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[PATCH] futex: Temporary disable FUTEX_PRIVATE_HASH

Sebastian Andrzej Siewior posted 1 patch 3 months, 1 week ago
There is a newer version of this series
init/Kconfig | 4 ++++
1 file changed, 4 insertions(+)
[PATCH] futex: Temporary disable FUTEX_PRIVATE_HASH
Posted by Sebastian Andrzej Siewior 3 months, 1 week ago 
Chris Mason reported a performance regression on big iron. Reports of
this kind were usually reported as part of a micro benchmark but Chris'
test did mimic his real workload. This makes it a real regression.

The root cause is rcuref_get() which is invoked during each futex
operation. If all threads of an application do this simultaneously then
it leads to cache line bouncing and the performance drops.

Disable FUTEX_PRIVATE_HASH entirely for this cycle. The performance
regression will be addressed in the following cycle enabling the option
again.

Reported-by: Chris Mason <clm@meta.com>
Closes: https://lore.kernel.org/all/3ad05298-351e-4d61-9972-ca45a0a50e33@meta.com/
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 init/Kconfig | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/init/Kconfig b/init/Kconfig
index af4c2f0854554..666783eb50abd 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1716,9 +1716,13 @@ config FUTEX_PI
 	depends on FUTEX && RT_MUTEXES
 	default y
 
+#
+# marked broken for performance reasons; gives us one more cycle to sort things out.
+#
 config FUTEX_PRIVATE_HASH
 	bool
 	depends on FUTEX && !BASE_SMALL && MMU
+	depends on BROKEN
 	default y
 
 config FUTEX_MPOL
-- 
2.50.0
Re: [PATCH] futex: Temporary disable FUTEX_PRIVATE_HASH
Posted by kernel test robot 2 months, 3 weeks ago 

Hello,


from the commit message, we know this 'temporary disable' is to address a
performance regression. so we still send out this report FYI what's the
possible performance impact. however, our team focus on micro benchmark,
so, anyway, just FYI.


kernel test robot noticed a 1.9% improvement of perf-bench-futex.ops/s on:


commit: bc1aa469e545fe16a62d501e095630cccc3fe1c4 ("[PATCH] futex: Temporary disable FUTEX_PRIVATE_HASH")
url: https://github.com/intel-lab-lkp/linux/commits/Sebastian-Andrzej-Siewior/futex-Temporary-disable-FUTEX_PRIVATE_HASH/20250630-225317
base: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git d0b3b7b22dfa1f4b515fd3a295b3fd958f9e81af
patch link: https://lore.kernel.org/all/20250630145034.8JnINEaS@linutronix.de/
patch subject: [PATCH] futex: Temporary disable FUTEX_PRIVATE_HASH

testcase: perf-bench-futex
config: x86_64-rhel-9.4
compiler: gcc-12
test machine: 192 threads 2 sockets Intel(R) Xeon(R) 6740E  CPU @ 2.4GHz (Sierra Forest) with 256G memory
parameters:

	runtime: 300s
	nr_task: 100%
	test: hash
	shared: shared
	cpufreq_governor: performance



Details are as below:
-------------------------------------------------------------------------------------------------->


The kernel config and materials to reproduce are available at:
https://download.01.org/0day-ci/archive/20250716/202507160223.2a483e8b-lkp@intel.com

=========================================================================================
compiler/cpufreq_governor/kconfig/nr_task/rootfs/runtime/shared/tbox_group/test/testcase:
  gcc-12/performance/x86_64-rhel-9.4/100%/debian-12-x86_64-20240206.cgz/300s/shared/lkp-srf-2sp2/hash/perf-bench-futex

commit: 
  v6.16-rc4
  bc1aa469e5 ("futex: Temporary disable FUTEX_PRIVATE_HASH")

       v6.16-rc4 bc1aa469e545fe16a62d501e095 
---------------- --------------------------- 
         %stddev     %change         %stddev
             \          |                \  
   2249734            +1.9%    2291792        perf-bench-futex.ops/s
      6622            +3.7%       6868        perf-bench-futex.time.user_time
    115.83 ± 12%     +40.1%     162.33 ± 18%  perf-sched.wait_and_delay.count.irqentry_exit_to_user_mode.asm_sysvec_call_function_single.[unknown].[unknown]
      4.40 ± 19%    +343.0%      19.51 ± 64%  perf-sched.wait_and_delay.max.ms.schedule_timeout.__wait_for_common.wait_for_completion_state.kernel_clone
      4.15 ± 17%    +347.3%      18.55 ± 69%  perf-sched.wait_time.max.ms.schedule_timeout.__wait_for_common.wait_for_completion_state.kernel_clone
     21.00 ± 63%    +121.4%      46.50 ± 11%  perf-c2c.DRAM.local
      4129 ± 57%    +225.5%      13444        perf-c2c.DRAM.remote
     67558 ± 56%     +66.3%     112351        perf-c2c.HITM.local
      4045 ± 57%    +227.2%      13237        perf-c2c.HITM.remote
     71603 ± 57%     +75.4%     125588        perf-c2c.HITM.total
 1.674e+08 ± 20%     +19.0%  1.991e+08        perf-stat.i.cache-misses
 4.096e+08 ± 20%     +19.4%  4.891e+08        perf-stat.i.cache-references
      0.36            +2.9%       0.37        perf-stat.overall.MPKI
      3127            -1.8%       3070        perf-stat.overall.cycles-between-cache-misses
 1.669e+08 ± 20%     +18.9%  1.985e+08        perf-stat.ps.cache-misses
 4.085e+08 ± 20%     +19.3%  4.874e+08        perf-stat.ps.cache-references
 1.622e+14            -1.0%  1.606e+14        perf-stat.total.instructions




Disclaimer:
Results have been estimated based on internal Intel analysis and are provided
for informational purposes only. Any difference in system hardware or software
design or configuration may affect actual performance.


-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki
[tip: locking/urgent] futex: Temporary disable FUTEX_PRIVATE_HASH
Posted by tip-bot2 for Sebastian Andrzej Siewior 3 months, 1 week ago 
The following commit has been merged into the locking/urgent branch of tip:

Commit-ID:     9a57c3773152a3ff2c35cc8325e088d011c9f83b
Gitweb:        https://git.kernel.org/tip/9a57c3773152a3ff2c35cc8325e088d011c9f83b
Author:        Sebastian Andrzej Siewior <bigeasy@linutronix.de>
AuthorDate:    Mon, 30 Jun 2025 16:50:34 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 01 Jul 2025 15:02:05 +02:00

futex: Temporary disable FUTEX_PRIVATE_HASH

Chris Mason reported a performance regression on big iron. Reports of
this kind were usually reported as part of a micro benchmark but Chris'
test did mimic his real workload. This makes it a real regression.

The root cause is rcuref_get() which is invoked during each futex
operation. If all threads of an application do this simultaneously then
it leads to cache line bouncing and the performance drops.

Disable FUTEX_PRIVATE_HASH entirely for this cycle. The performance
regression will be addressed in the following cycle enabling the option
again.

Closes: https://lore.kernel.org/all/3ad05298-351e-4d61-9972-ca45a0a50e33@meta.com/
Reported-by: Chris Mason <clm@meta.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250630145034.8JnINEaS@linutronix.de
---
 init/Kconfig | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/init/Kconfig b/init/Kconfig
index af4c2f0..666783e 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1716,9 +1716,13 @@ config FUTEX_PI
 	depends on FUTEX && RT_MUTEXES
 	default y
 
+#
+# marked broken for performance reasons; gives us one more cycle to sort things out.
+#
 config FUTEX_PRIVATE_HASH
 	bool
 	depends on FUTEX && !BASE_SMALL && MMU
+	depends on BROKEN
 	default y
 
 config FUTEX_MPOL

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