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Message-ID: <99b39d9e-1fae-41a5-96f9-d1298c7cc29c@igalia.com>
Date: Thu, 18 Apr 2024 09:54:08 +0100
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Subject: Re: [PATCH] sched/psi: Optimise psi_group_change a bit
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To: Tvrtko Ursulin <tursulin@igalia.com>, Ingo Molnar <mingo@kernel.org>
Cc: linux-kernel@vger.kernel.org, Tvrtko Ursulin <tursulin@ursulin.net>,
 Suren Baghdasaryan <surenb@google.com>, Peter Ziljstra
 <peterz@infradead.org>, kernel-dev@igalia.com,
 Johannes Weiner <hannes@cmpxchg.org>
References: <20240329160648.86999-1-tursulin@igalia.com>
 <20240329185147.GA877460@cmpxchg.org> <20240409223847.GE1057805@cmpxchg.org>
From: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
In-Reply-To: <20240409223847.GE1057805@cmpxchg.org>
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Hi Ingo,

On 09/04/2024 23:38, Johannes Weiner wrote:
> [ Oops, I still had an old mutt alias for Ingo's address. ]
> 
> Ingo, would you mind taking this through the scheduler tree?

Gentle reminder so this one does not fall through the cracks.

Regards,

Tvrtko

> On Fri, Mar 29, 2024 at 02:51:53PM -0400, Johannes Weiner wrote:
>> 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
>>>