Re: [PATCH] sched/psi: Optimise psi_group_change a bit

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Mar 29, 2024 at 04:06:48PM +0000, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tursulin@ursulin.net>
> 
> The current code loops over the psi_states only to call a helper which
> then resolves back to the action needed for each state using a switch
> statement. That is effectively creating a double indirection of a kind
> which, given how all the states need to be explicitly listed and handled
> anyway, we can simply remove. Both the for loop and the switch statement
> that is.
> 
> The benefit is both in the code size and CPU time spent in this function.
> YMMV but on my Steam Deck, while in a game, the patch makes the CPU usage
> go from ~2.4% down to ~1.2%. Text size at the same time went from 0x323 to
> 0x2c1.
> 
> Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Cc: Peter Ziljstra <peterz@infradead.org>
> Cc: linux-kernel@vger.kernel.org
> Cc: kernel-dev@igalia.com

This is great.

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

Ingo, would you mind please taking this through the scheduler tree? I
think Peter is still out.

Remaining quote below.

Thanks

> ---
>  kernel/sched/psi.c | 54 +++++++++++++++++++++++-----------------------
>  1 file changed, 27 insertions(+), 27 deletions(-)
> 
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index 7b4aa5809c0f..55720ecf420e 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -218,28 +218,32 @@ void __init psi_init(void)
>  	group_init(&psi_system);
>  }
>  
> -static bool test_state(unsigned int *tasks, enum psi_states state, bool oncpu)
> +static u32 test_states(unsigned int *tasks, u32 state_mask)
>  {
> -	switch (state) {
> -	case PSI_IO_SOME:
> -		return unlikely(tasks[NR_IOWAIT]);
> -	case PSI_IO_FULL:
> -		return unlikely(tasks[NR_IOWAIT] && !tasks[NR_RUNNING]);
> -	case PSI_MEM_SOME:
> -		return unlikely(tasks[NR_MEMSTALL]);
> -	case PSI_MEM_FULL:
> -		return unlikely(tasks[NR_MEMSTALL] &&
> -			tasks[NR_RUNNING] == tasks[NR_MEMSTALL_RUNNING]);
> -	case PSI_CPU_SOME:
> -		return unlikely(tasks[NR_RUNNING] > oncpu);
> -	case PSI_CPU_FULL:
> -		return unlikely(tasks[NR_RUNNING] && !oncpu);
> -	case PSI_NONIDLE:
> -		return tasks[NR_IOWAIT] || tasks[NR_MEMSTALL] ||
> -			tasks[NR_RUNNING];
> -	default:
> -		return false;
> +	const bool oncpu = state_mask & PSI_ONCPU;
> +
> +	if (tasks[NR_IOWAIT]) {
> +		state_mask |= BIT(PSI_IO_SOME);
> +		if (!tasks[NR_RUNNING])
> +			state_mask |= BIT(PSI_IO_FULL);
>  	}
> +
> +	if (tasks[NR_MEMSTALL]) {
> +		state_mask |= BIT(PSI_MEM_SOME);
> +		if (tasks[NR_RUNNING] == tasks[NR_MEMSTALL_RUNNING])
> +			state_mask |= BIT(PSI_MEM_FULL);
> +	}
> +
> +	if (tasks[NR_RUNNING] > oncpu)
> +		state_mask |= BIT(PSI_CPU_SOME);
> +
> +	if (tasks[NR_RUNNING] && !oncpu)
> +		state_mask |= BIT(PSI_CPU_FULL);
> +
> +	if (tasks[NR_IOWAIT] || tasks[NR_MEMSTALL] || tasks[NR_RUNNING])
> +		state_mask |= BIT(PSI_NONIDLE);
> +
> +	return state_mask;
>  }
>  
>  static void get_recent_times(struct psi_group *group, int cpu,
> @@ -770,7 +774,6 @@ static void psi_group_change(struct psi_group *group, int cpu,
>  {
>  	struct psi_group_cpu *groupc;
>  	unsigned int t, m;
> -	enum psi_states s;
>  	u32 state_mask;
>  
>  	groupc = per_cpu_ptr(group->pcpu, cpu);
> @@ -841,10 +844,7 @@ static void psi_group_change(struct psi_group *group, int cpu,
>  		return;
>  	}
>  
> -	for (s = 0; s < NR_PSI_STATES; s++) {
> -		if (test_state(groupc->tasks, s, state_mask & PSI_ONCPU))
> -			state_mask |= (1 << s);
> -	}
> +	state_mask = test_states(groupc->tasks, state_mask);
>  
>  	/*
>  	 * Since we care about lost potential, a memstall is FULL
> @@ -1194,7 +1194,7 @@ void psi_cgroup_restart(struct psi_group *group)
>  	/*
>  	 * After we disable psi_group->enabled, we don't actually
>  	 * stop percpu tasks accounting in each psi_group_cpu,
> -	 * instead only stop test_state() loop, record_times()
> +	 * instead only stop test_states() loop, record_times()
>  	 * and averaging worker, see psi_group_change() for details.
>  	 *
>  	 * When disable cgroup PSI, this function has nothing to sync
> @@ -1202,7 +1202,7 @@ void psi_cgroup_restart(struct psi_group *group)
>  	 * would see !psi_group->enabled and only do task accounting.
>  	 *
>  	 * When re-enable cgroup PSI, this function use psi_group_change()
> -	 * to get correct state mask from test_state() loop on tasks[],
> +	 * to get correct state mask from test_states() loop on tasks[],
>  	 * and restart groupc->state_start from now, use .clear = .set = 0
>  	 * here since no task status really changed.
>  	 */
> -- 
> 2.44.0
>