Re: [PATCH v2] sched: Move psi_account_irqtime() out of update_rq_clock_task() hotpath

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Jun 20, 2024 at 10:32:57AM -0700, John Stultz wrote:
> 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
> 
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Ben Segall <bsegall@google.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
> Cc: Valentin Schneider <vschneid@redhat.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Cc: Chengming Zhou <zhouchengming@bytedance.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Cc: Qais Yousef <qyousef@layalina.io>
> Cc: Joel Fernandes <joel@joelfernandes.org>
> Cc: kernel-team@android.com
> Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
> Reviewed-by: Qais Yousef <qyousef@layalina.io>
> Fixes: 52b1364ba0b1 ("sched/psi: Add PSI_IRQ to track IRQ/SOFTIRQ pressure")
> Originally-by: Peter Zijlstra <peterz@infradead.org>
> Reported-by: Jimmy Shiu <jimmyshiu@google.com>
> Signed-off-by: John Stultz <jstultz@google.com>

Excellent.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>