Re: [PATCH 6.1] sched: Move psi_account_irqtime() out of update_rq_clock_task() hotpath

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Jul 15, 2024 at 05:40:39PM -0700, John Stultz wrote:
> commit ddae0ca2a8fe12d0e24ab10ba759c3fbd755ada8 upstream.
> 
> It was reported that in moving to 6.1, a larger then 10%
> regression was seen in the performance of
> clock_gettime(CLOCK_THREAD_CPUTIME_ID,...).
> 
> Using a simple reproducer, I found:
> 5.10:
> 100000000 calls in 24345994193 ns => 243.460 ns per call
> 100000000 calls in 24288172050 ns => 242.882 ns per call
> 100000000 calls in 24289135225 ns => 242.891 ns per call
> 
> 6.1:
> 100000000 calls in 28248646742 ns => 282.486 ns per call
> 100000000 calls in 28227055067 ns => 282.271 ns per call
> 100000000 calls in 28177471287 ns => 281.775 ns per call
> 
> The cause of this was finally narrowed down to the addition of
> psi_account_irqtime() in update_rq_clock_task(), in commit
> 52b1364ba0b1 ("sched/psi: Add PSI_IRQ to track IRQ/SOFTIRQ
> pressure").
> 
> In my initial attempt to resolve this, I leaned towards moving
> all accounting work out of the clock_gettime() call path, but it
> wasn't very pretty, so it will have to wait for a later deeper
> rework. Instead, Peter shared this approach:
> 
> Rework psi_account_irqtime() to use its own psi_irq_time base
> for accounting, and move it out of the hotpath, calling it
> instead from sched_tick() and __schedule().
> 
> In testing this, we found the importance of ensuring
> psi_account_irqtime() is run under the rq_lock, which Johannes
> Weiner helpfully explained, so also add some lockdep annotations
> to make that requirement clear.
> 
> With this change the performance is back in-line with 5.10:
> 6.1+fix:
> 100000000 calls in 24297324597 ns => 242.973 ns per call
> 100000000 calls in 24318869234 ns => 243.189 ns per call
> 100000000 calls in 24291564588 ns => 242.916 ns per call
> 
> Reported-by: Jimmy Shiu <jimmyshiu@google.com>
> Originally-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: John Stultz <jstultz@google.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
> Reviewed-by: Qais Yousef <qyousef@layalina.io>
> Link: https://lore.kernel.org/r/20240618215909.4099720-1-jstultz@google.com
> Fixes: 52b1364ba0b1 ("sched/psi: Add PSI_IRQ to track IRQ/SOFTIRQ pressure")
> [jstultz: Fixed up minor collisions w/ 6.1-stable]
> Signed-off-by: John Stultz <jstultz@google.com>
> ---
>  kernel/sched/core.c  |  7 +++++--
>  kernel/sched/psi.c   | 21 ++++++++++++++++-----
>  kernel/sched/sched.h |  1 +
>  kernel/sched/stats.h | 11 ++++++++---
>  4 files changed, 30 insertions(+), 10 deletions(-)

Both backports now queued up, thanks.

greg k-h