Lazy preemption bits

[PATCH 1/7] sched: warn for high latency with TIF_NEED_RESCHED_LAZY

Posted by Ankur Arora 1 year, 4 months ago

resched_latency_warn() now also warns if TIF_NEED_RESCHED_LAZY is set
without rescheduling for more than the latency_warn_ms period.

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Ziljstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
 kernel/sched/core.c  | 2 +-
 kernel/sched/debug.c | 7 +++++--
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 694bfcf153cb..1229766b704e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5571,7 +5571,7 @@ static u64 cpu_resched_latency(struct rq *rq)
 	if (sysctl_resched_latency_warn_once && warned_once)
 		return 0;
 
-	if (!need_resched() || !latency_warn_ms)
+	if ((!need_resched() && !tif_need_resched_lazy()) || !latency_warn_ms)
 		return 0;
 
 	if (system_state == SYSTEM_BOOTING)
diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 9abcc6ead11b..f0d551ba64bb 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -1293,9 +1293,12 @@ void proc_sched_set_task(struct task_struct *p)
 void resched_latency_warn(int cpu, u64 latency)
 {
 	static DEFINE_RATELIMIT_STATE(latency_check_ratelimit, 60 * 60 * HZ, 1);
+	char *nr;
+
+	nr = tif_need_resched() ? "need_resched" : "need_resched_lazy";
 
 	WARN(__ratelimit(&latency_check_ratelimit),
-	     "sched: CPU %d need_resched set for > %llu ns (%d ticks) "
+	     "sched: CPU %d %s set for > %llu ns (%d ticks) "
 	     "without schedule\n",
-	     cpu, latency, cpu_rq(cpu)->ticks_without_resched);
+	     cpu, nr, latency, cpu_rq(cpu)->ticks_without_resched);
 }
-- 
2.43.5

Re: [PATCH 1/7] sched: warn for high latency with TIF_NEED_RESCHED_LAZY

Posted by Shrikanth Hegde 1 year, 3 months ago


On 10/9/24 22:24, Ankur Arora wrote:
> resched_latency_warn() now also warns if TIF_NEED_RESCHED_LAZY is set
> without rescheduling for more than the latency_warn_ms period.
>

I am bit confused here. Why do we need to warn if LAZY is set for a long 
time?

If lazy set, the subsequent tick, it would be set to upgraded to 
NEED_RESCHED.

Since the value of latency_warn_ms=100ms, that means even on system with 
HZ=100, that means 10 ticks before that warning would be printed no?


IIUC, the changelog c006fac556e40 ("sched: Warn on long periods of 
pending need_resched") has the concern of need_resched set but if it is 
non-preemptible kernel it would spend a lot of time in kernel mode. In 
that case print a warning.

If someone enables Lazy, that means it is preemptible and probably this 
whole notion of resched_latency_warn doesn't apply to lazy. Please 
correct me if i am not understanding this correctly.

> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Ziljstra <peterz@infradead.org>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
> ---
>   kernel/sched/core.c  | 2 +-
>   kernel/sched/debug.c | 7 +++++--
>   2 files changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 694bfcf153cb..1229766b704e 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5571,7 +5571,7 @@ static u64 cpu_resched_latency(struct rq *rq)
>   	if (sysctl_resched_latency_warn_once && warned_once)
>   		return 0;
>   
> -	if (!need_resched() || !latency_warn_ms)
> +	if ((!need_resched() && !tif_need_resched_lazy()) || !latency_warn_ms)
>   		return 0;
>   
>   	if (system_state == SYSTEM_BOOTING)
> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> index 9abcc6ead11b..f0d551ba64bb 100644
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -1293,9 +1293,12 @@ void proc_sched_set_task(struct task_struct *p)
>   void resched_latency_warn(int cpu, u64 latency)
>   {
>   	static DEFINE_RATELIMIT_STATE(latency_check_ratelimit, 60 * 60 * HZ, 1);
> +	char *nr;
> +
> +	nr = tif_need_resched() ? "need_resched" : "need_resched_lazy";
>   
>   	WARN(__ratelimit(&latency_check_ratelimit),
> -	     "sched: CPU %d need_resched set for > %llu ns (%d ticks) "
> +	     "sched: CPU %d %s set for > %llu ns (%d ticks) "
>   	     "without schedule\n",
> -	     cpu, latency, cpu_rq(cpu)->ticks_without_resched);
> +	     cpu, nr, latency, cpu_rq(cpu)->ticks_without_resched);
>   }

Re: [PATCH 1/7] sched: warn for high latency with TIF_NEED_RESCHED_LAZY

Posted by Ankur Arora 1 year, 3 months ago

Shrikanth Hegde <sshegde@linux.ibm.com> writes:

> On 10/9/24 22:24, Ankur Arora wrote:
>> resched_latency_warn() now also warns if TIF_NEED_RESCHED_LAZY is set
>> without rescheduling for more than the latency_warn_ms period.
>>
>
> I am bit confused here. Why do we need to warn if LAZY is set for a long time?
>
> If lazy set, the subsequent tick, it would be set to upgraded to NEED_RESCHED.
>
> Since the value of latency_warn_ms=100ms, that means even on system with HZ=100,
> that means 10 ticks before that warning would be printed no?

That's a fair point. However, the assumption there is that there are no
bugs in upgrade on tick or that there's no situation in which the tick
is off for a prolonged period.

Ankur

> IIUC, the changelog c006fac556e40 ("sched: Warn on long periods of pending
> need_resched") has the concern of need_resched set but if it is non-preemptible
> kernel it would spend a lot of time in kernel mode. In that case print a
> warning.
>
> If someone enables Lazy, that means it is preemptible and probably this whole
> notion of resched_latency_warn doesn't apply to lazy. Please correct me if i am
> not understanding this correctly.
>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: Peter Ziljstra <peterz@infradead.org>
>> Cc: Juri Lelli <juri.lelli@redhat.com>
>> Cc: Vincent Guittot <vincent.guittot@linaro.org>
>> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
>> ---
>>   kernel/sched/core.c  | 2 +-
>>   kernel/sched/debug.c | 7 +++++--
>>   2 files changed, 6 insertions(+), 3 deletions(-)
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 694bfcf153cb..1229766b704e 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -5571,7 +5571,7 @@ static u64 cpu_resched_latency(struct rq *rq)
>>   	if (sysctl_resched_latency_warn_once && warned_once)
>>   		return 0;
>>   -	if (!need_resched() || !latency_warn_ms)
>> +	if ((!need_resched() && !tif_need_resched_lazy()) || !latency_warn_ms)
>>   		return 0;
>>     	if (system_state == SYSTEM_BOOTING)
>> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
>> index 9abcc6ead11b..f0d551ba64bb 100644
>> --- a/kernel/sched/debug.c
>> +++ b/kernel/sched/debug.c
>> @@ -1293,9 +1293,12 @@ void proc_sched_set_task(struct task_struct *p)
>>   void resched_latency_warn(int cpu, u64 latency)
>>   {
>>   	static DEFINE_RATELIMIT_STATE(latency_check_ratelimit, 60 * 60 * HZ, 1);
>> +	char *nr;
>> +
>> +	nr = tif_need_resched() ? "need_resched" : "need_resched_lazy";
>>     	WARN(__ratelimit(&latency_check_ratelimit),
>> -	     "sched: CPU %d need_resched set for > %llu ns (%d ticks) "
>> +	     "sched: CPU %d %s set for > %llu ns (%d ticks) "
>>   	     "without schedule\n",
>> -	     cpu, latency, cpu_rq(cpu)->ticks_without_resched);
>> +	     cpu, nr, latency, cpu_rq(cpu)->ticks_without_resched);
>>   }


--
ankur

Re: [PATCH 1/7] sched: warn for high latency with TIF_NEED_RESCHED_LAZY

Posted by Shrikanth Hegde 1 year, 3 months ago


On 10/22/24 00:51, Ankur Arora wrote:
> 
> Shrikanth Hegde <sshegde@linux.ibm.com> writes:
> 
>> On 10/9/24 22:24, Ankur Arora wrote:
>>> resched_latency_warn() now also warns if TIF_NEED_RESCHED_LAZY is set
>>> without rescheduling for more than the latency_warn_ms period.
>>>
>>
>> I am bit confused here. Why do we need to warn if LAZY is set for a long time?
>>
>> If lazy set, the subsequent tick, it would be set to upgraded to NEED_RESCHED.
>>
>> Since the value of latency_warn_ms=100ms, that means even on system with HZ=100,
>> that means 10 ticks before that warning would be printed no?
> 
> That's a fair point. However, the assumption there is that there are no
> bugs in upgrade on tick or that there's no situation in which the tick
> is off for a prolonged period.
> 

ok.

But if tick is off, then ticks_without_resched isn't incremented either. 
IIUC, this check is for situation when NR is set and tick is on.

> Ankur
> 
>> IIUC, the changelog c006fac556e40 ("sched: Warn on long periods of pending
>> need_resched") has the concern of need_resched set but if it is non-preemptible
>> kernel it would spend a lot of time in kernel mode. In that case print a
>> warning.
>>
>> If someone enables Lazy, that means it is preemptible and probably this whole
>> notion of resched_latency_warn doesn't apply to lazy. Please correct me if i am
>> not understanding this correctly.
>>
>>> Cc: Ingo Molnar <mingo@redhat.com>
>>> Cc: Peter Ziljstra <peterz@infradead.org>
>>> Cc: Juri Lelli <juri.lelli@redhat.com>
>>> Cc: Vincent Guittot <vincent.guittot@linaro.org>
>>> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
>>> ---
>>>    kernel/sched/core.c  | 2 +-
>>>    kernel/sched/debug.c | 7 +++++--
>>>    2 files changed, 6 insertions(+), 3 deletions(-)
>>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>>> index 694bfcf153cb..1229766b704e 100644
>>> --- a/kernel/sched/core.c
>>> +++ b/kernel/sched/core.c
>>> @@ -5571,7 +5571,7 @@ static u64 cpu_resched_latency(struct rq *rq)
>>>    	if (sysctl_resched_latency_warn_once && warned_once)
>>>    		return 0;
>>>    -	if (!need_resched() || !latency_warn_ms)
>>> +	if ((!need_resched() && !tif_need_resched_lazy()) || !latency_warn_ms)
>>>    		return 0;
>>>      	if (system_state == SYSTEM_BOOTING)
>>> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
>>> index 9abcc6ead11b..f0d551ba64bb 100644
>>> --- a/kernel/sched/debug.c
>>> +++ b/kernel/sched/debug.c
>>> @@ -1293,9 +1293,12 @@ void proc_sched_set_task(struct task_struct *p)
>>>    void resched_latency_warn(int cpu, u64 latency)
>>>    {
>>>    	static DEFINE_RATELIMIT_STATE(latency_check_ratelimit, 60 * 60 * HZ, 1);
>>> +	char *nr;
>>> +
>>> +	nr = tif_need_resched() ? "need_resched" : "need_resched_lazy";
>>>      	WARN(__ratelimit(&latency_check_ratelimit),
>>> -	     "sched: CPU %d need_resched set for > %llu ns (%d ticks) "
>>> +	     "sched: CPU %d %s set for > %llu ns (%d ticks) "
>>>    	     "without schedule\n",
>>> -	     cpu, latency, cpu_rq(cpu)->ticks_without_resched);
>>> +	     cpu, nr, latency, cpu_rq(cpu)->ticks_without_resched);
>>>    }
> 
> 
> --
> ankur

Re: [PATCH 1/7] sched: warn for high latency with TIF_NEED_RESCHED_LAZY

Posted by kernel test robot 1 year, 3 months ago

Hi Ankur,

kernel test robot noticed the following build errors:

[auto build test ERROR on paulmck-rcu/dev]
[also build test ERROR on powerpc/next powerpc/fixes tip/sched/core linus/master v6.12-rc2 next-20241011]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Ankur-Arora/sched-warn-for-high-latency-with-TIF_NEED_RESCHED_LAZY/20241010-005819
base:   https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git dev
patch link:    https://lore.kernel.org/r/20241009165411.3426937-2-ankur.a.arora%40oracle.com
patch subject: [PATCH 1/7] sched: warn for high latency with TIF_NEED_RESCHED_LAZY
config: x86_64-kexec (https://download.01.org/0day-ci/archive/20241013/202410131726.Gi9qvUP8-lkp@intel.com/config)
compiler: clang version 18.1.8 (https://github.com/llvm/llvm-project 3b5b5c1ec4a3095ab096dd780e84d7ab81f3d7ff)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20241013/202410131726.Gi9qvUP8-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202410131726.Gi9qvUP8-lkp@intel.com/

All errors (new ones prefixed by >>):

>> kernel/sched/core.c:5528:27: error: call to undeclared function 'tif_need_resched_lazy'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    5528 |         if ((!need_resched() && !tif_need_resched_lazy()) || !latency_warn_ms)
         |                                  ^
   kernel/sched/core.c:5528:27: note: did you mean 'tif_need_resched'?
   include/linux/thread_info.h:182:29: note: 'tif_need_resched' declared here
     182 | static __always_inline bool tif_need_resched(void)
         |                             ^
   1 error generated.


vim +/tif_need_resched_lazy +5528 kernel/sched/core.c

  5517	
  5518	#ifdef CONFIG_SCHED_DEBUG
  5519	static u64 cpu_resched_latency(struct rq *rq)
  5520	{
  5521		int latency_warn_ms = READ_ONCE(sysctl_resched_latency_warn_ms);
  5522		u64 resched_latency, now = rq_clock(rq);
  5523		static bool warned_once;
  5524	
  5525		if (sysctl_resched_latency_warn_once && warned_once)
  5526			return 0;
  5527	
> 5528		if ((!need_resched() && !tif_need_resched_lazy()) || !latency_warn_ms)
  5529			return 0;
  5530	
  5531		if (system_state == SYSTEM_BOOTING)
  5532			return 0;
  5533	
  5534		if (!rq->last_seen_need_resched_ns) {
  5535			rq->last_seen_need_resched_ns = now;
  5536			rq->ticks_without_resched = 0;
  5537			return 0;
  5538		}
  5539	
  5540		rq->ticks_without_resched++;
  5541		resched_latency = now - rq->last_seen_need_resched_ns;
  5542		if (resched_latency <= latency_warn_ms * NSEC_PER_MSEC)
  5543			return 0;
  5544	
  5545		warned_once = true;
  5546	
  5547		return resched_latency;
  5548	}
  5549	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH 1/7] sched: warn for high latency with TIF_NEED_RESCHED_LAZY

Posted by kernel test robot 1 year, 3 months ago

Hi Ankur,

kernel test robot noticed the following build errors:

[auto build test ERROR on paulmck-rcu/dev]
[also build test ERROR on powerpc/next powerpc/fixes tip/sched/core linus/master v6.12-rc2 next-20241011]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Ankur-Arora/sched-warn-for-high-latency-with-TIF_NEED_RESCHED_LAZY/20241010-005819
base:   https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git dev
patch link:    https://lore.kernel.org/r/20241009165411.3426937-2-ankur.a.arora%40oracle.com
patch subject: [PATCH 1/7] sched: warn for high latency with TIF_NEED_RESCHED_LAZY
config: x86_64-rhel-8.3 (https://download.01.org/0day-ci/archive/20241013/202410131715.HYC3WK5i-lkp@intel.com/config)
compiler: gcc-12 (Debian 12.2.0-14) 12.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20241013/202410131715.HYC3WK5i-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202410131715.HYC3WK5i-lkp@intel.com/

All errors (new ones prefixed by >>):

   kernel/sched/core.c: In function 'cpu_resched_latency':
>> kernel/sched/core.c:5528:34: error: implicit declaration of function 'tif_need_resched_lazy'; did you mean 'tif_need_resched'? [-Werror=implicit-function-declaration]
    5528 |         if ((!need_resched() && !tif_need_resched_lazy()) || !latency_warn_ms)
         |                                  ^~~~~~~~~~~~~~~~~~~~~
         |                                  tif_need_resched
   cc1: some warnings being treated as errors


vim +5528 kernel/sched/core.c

  5517	
  5518	#ifdef CONFIG_SCHED_DEBUG
  5519	static u64 cpu_resched_latency(struct rq *rq)
  5520	{
  5521		int latency_warn_ms = READ_ONCE(sysctl_resched_latency_warn_ms);
  5522		u64 resched_latency, now = rq_clock(rq);
  5523		static bool warned_once;
  5524	
  5525		if (sysctl_resched_latency_warn_once && warned_once)
  5526			return 0;
  5527	
> 5528		if ((!need_resched() && !tif_need_resched_lazy()) || !latency_warn_ms)
  5529			return 0;
  5530	
  5531		if (system_state == SYSTEM_BOOTING)
  5532			return 0;
  5533	
  5534		if (!rq->last_seen_need_resched_ns) {
  5535			rq->last_seen_need_resched_ns = now;
  5536			rq->ticks_without_resched = 0;
  5537			return 0;
  5538		}
  5539	
  5540		rq->ticks_without_resched++;
  5541		resched_latency = now - rq->last_seen_need_resched_ns;
  5542		if (resched_latency <= latency_warn_ms * NSEC_PER_MSEC)
  5543			return 0;
  5544	
  5545		warned_once = true;
  5546	
  5547		return resched_latency;
  5548	}
  5549	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH 1/7] sched: warn for high latency with TIF_NEED_RESCHED_LAZY

Posted by Sebastian Andrzej Siewior 1 year, 4 months ago

On 2024-10-09 09:54:05 [-0700], Ankur Arora wrote:
> resched_latency_warn() now also warns if TIF_NEED_RESCHED_LAZY is set
> without rescheduling for more than the latency_warn_ms period.

The description is odd. I think you want to say that
resched_latency_warn() does not warn if a task has TIF_NEED_RESCHED_LAZY
has set for longer periods and you want to add this functionality.

> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Ziljstra <peterz@infradead.org>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
…
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 694bfcf153cb..1229766b704e 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5571,7 +5571,7 @@ static u64 cpu_resched_latency(struct rq *rq)
>  	if (sysctl_resched_latency_warn_once && warned_once)
>  		return 0;
>  
> -	if (!need_resched() || !latency_warn_ms)
> +	if ((!need_resched() && !tif_need_resched_lazy()) || !latency_warn_ms)

tif_need_resched_lazy() is not doing what you think it is doing.

Either PeterZ makes a helper for this or you need
tif_test_bit(TIF_NEED_RESCHED_LAZY).

>  		return 0;
>  
>  	if (system_state == SYSTEM_BOOTING)

Sebastian

Re: [PATCH 1/7] sched: warn for high latency with TIF_NEED_RESCHED_LAZY

Posted by Ankur Arora 1 year, 4 months ago

Sebastian Andrzej Siewior <bigeasy@linutronix.de> writes:

> On 2024-10-09 09:54:05 [-0700], Ankur Arora wrote:
>> resched_latency_warn() now also warns if TIF_NEED_RESCHED_LAZY is set
>> without rescheduling for more than the latency_warn_ms period.
>
> The description is odd. I think you want to say that
> resched_latency_warn() does not warn if a task has TIF_NEED_RESCHED_LAZY
> has set for longer periods and you want to add this functionality.
>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: Peter Ziljstra <peterz@infradead.org>
>> Cc: Juri Lelli <juri.lelli@redhat.com>
>> Cc: Vincent Guittot <vincent.guittot@linaro.org>
>> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
> …
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 694bfcf153cb..1229766b704e 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -5571,7 +5571,7 @@ static u64 cpu_resched_latency(struct rq *rq)
>>  	if (sysctl_resched_latency_warn_once && warned_once)
>>  		return 0;
>>
>> -	if (!need_resched() || !latency_warn_ms)
>> +	if ((!need_resched() && !tif_need_resched_lazy()) || !latency_warn_ms)
>
> tif_need_resched_lazy() is not doing what you think it is doing.

Thanks. My bad.

> tif_test_bit(TIF_NEED_RESCHED_LAZY).

> Either PeterZ makes a helper for this or you need
> tif_test_bit(TIF_NEED_RESCHED_LAZY).

Yeah. tif_test_bit(TIF_NEED_RESCHED_LAZY) and need_resched() will
be identical with !ARCH_HAS_PREEMPT_LAZY but that should be okay in
this context.

--
ankur