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From: Valentin Schneider <valentin.schneider@arm.com>
To: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org,
	peterz@infradead.org, vincent.guittot@linaro.org
Subject: Re: [PATCH v2 9/9] sched/topology: Define and use shortcut pointers for wakeup sd_flag scan
Date: Thu, 19 Mar 2020 12:22:34 +0000	[thread overview]
Message-ID: <jhj8sjwilf9.mognet@arm.com> (raw)
In-Reply-To: <53763a32-0ce5-e267-9d5d-99e65c921d08@arm.com>


On Thu, Mar 19 2020, Dietmar Eggemann wrote:
> On 11.03.20 19:16, Valentin Schneider wrote:
>> @@ -6621,7 +6610,19 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>>
>>      rcu_read_lock();
>>
>> -	sd = highest_flag_domain(cpu, sd_flag);
>> +	switch (wake_flags & (WF_TTWU | WF_FORK | WF_EXEC)) {
>> +	case WF_TTWU:
>> +		sd_flag = SD_BALANCE_WAKE;
>> +		sd = rcu_dereference(per_cpu(sd_balance_wake, cpu));
>
> IMHO, since we hard-code 0*SD_BALANCE_WAKE in sd_init(), sd would always
> be NULL, so !want_affine (i.e. wake_wide()) would still go sis().
>
> SD_BALANCE_WAKE is no a topology related sd_flag so it can't be set from
> outside. Since the sd->flags sysctl is now read-only, wouldn't this case
> be redundant?
>

On a purely mainline kernel, yes, 'sd' will always be NULL for
wakeups. I'm playing a bit more conservative here with SD_BALANCE_WAKE,
as I could see some folks using that flag in some ~franken~ downstream
kernels (I am not aware of any, however).

Also, to be fair we've only very recently gotten rid of the last
SD_BALANCE_WAKE user (asymmetric CPU capacity topologies), so I felt
like keeping it around for a bit before entirely killing it would be a
sane thing to do.

> [...]

      reply	other threads:[~2020-03-19 12:23 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-11 18:15 [PATCH v2 0/9] sched: Streamline select_task_rq() & select_task_rq_fair() Valentin Schneider
2020-03-11 18:15 ` [PATCH v2 1/9] sched/fair: find_idlest_group(): Remove unused sd_flag parameter Valentin Schneider
2020-03-19  9:05   ` Dietmar Eggemann
2020-03-11 18:15 ` [PATCH v2 2/9] sched/debug: Make sd->flags sysctl read-only Valentin Schneider
2020-03-19  9:07   ` Dietmar Eggemann
2020-03-19 12:04     ` Valentin Schneider
2020-03-11 18:15 ` [PATCH v2 3/9] sched: Remove checks against SD_LOAD_BALANCE Valentin Schneider
2020-03-19 10:28   ` Dietmar Eggemann
2020-03-19 12:05     ` Valentin Schneider
2020-03-23 14:26       ` Dietmar Eggemann
2020-03-23 17:17         ` Valentin Schneider
2020-03-11 18:15 ` [PATCH v2 4/9] sched/topology: Kill SD_LOAD_BALANCE Valentin Schneider
2020-03-19 10:29   ` Dietmar Eggemann
2020-03-19 12:06     ` Valentin Schneider
2020-03-11 18:15 ` [PATCH v2 5/9] sched: Add WF_TTWU, WF_EXEC wakeup flags Valentin Schneider
2020-03-11 18:15 ` [PATCH v2 6/9] sched: Kill select_task_rq()'s sd_flag parameter Valentin Schneider
2020-03-11 18:15 ` [PATCH v2 7/9] sched/fair: Dissociate wakeup decisions from SD flag value Valentin Schneider
2020-03-11 18:16 ` [PATCH v2 8/9] sched/fair: Split select_task_rq_fair want_affine logic Valentin Schneider
2020-03-19 10:30   ` Dietmar Eggemann
2020-03-19 12:06     ` Valentin Schneider
2020-03-11 18:16 ` [PATCH v2 9/9] sched/topology: Define and use shortcut pointers for wakeup sd_flag scan Valentin Schneider
2020-03-19 10:46   ` Dietmar Eggemann
2020-03-19 12:22     ` Valentin Schneider [this message]

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