nohz_full is a feature that only fits into rare and very corner cases.
Yet distros enable it by default and therefore the related fields are
always reserved in the task struct.

Those task fields are stored in the middle of cacheline hot places such
as cputime accounting and context switch counting, which doesn't make
any sense for a feature that is disabled most of the time.

Move the nohz_full storage to colder places.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 include/linux/sched.h | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4f78a64beb52..117aa20b8fb6 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1114,13 +1114,7 @@ struct task_struct {
 #endif
 	u64				gtime;
 	struct prev_cputime		prev_cputime;
-#ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
-	struct vtime			vtime;
-#endif
 
-#ifdef CONFIG_NO_HZ_FULL
-	atomic_t			tick_dep_mask;
-#endif
 	/* Context switch counts: */
 	unsigned long			nvcsw;
 	unsigned long			nivcsw;
@@ -1446,6 +1440,14 @@ struct task_struct {
 	struct task_delay_info		*delays;
 #endif
 
+#ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
+	struct vtime			vtime;
+#endif
+
+#ifdef CONFIG_NO_HZ_FULL
+	atomic_t			tick_dep_mask;
+#endif
+
 #ifdef CONFIG_FAULT_INJECTION
 	int				make_it_fail;
 	unsigned int			fail_nth;
-- 
2.48.1

On 03/07/25 16:07, Frederic Weisbecker wrote:
> nohz_full is a feature that only fits into rare and very corner cases.
> Yet distros enable it by default and therefore the related fields are
> always reserved in the task struct.
>
> Those task fields are stored in the middle of cacheline hot places such
> as cputime accounting and context switch counting, which doesn't make
> any sense for a feature that is disabled most of the time.
>
> Move the nohz_full storage to colder places.
>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

task_struct is still a bloody maze to me, but AFAICT there's *at least one*
full non-optional cacheline - the signal handling faff - between cputime
accounting and this new location, so:

Reviewed-by: Valentin Schneider <vschneid@redhat.com>