On 28/04/2021 15:04, Vincent Guittot wrote:
On Wed, 28 Apr 2021 at 11:51, Song Bao Hua (Barry Song) song.bao.hua@hisilicon.com wrote:
-----Original Message----- From: Dietmar Eggemann [mailto:dietmar.eggemann@arm.com]
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On 20/04/2021 02:18, Barry Song wrote:
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I am really confused. The whole code has only checked if wake_flags has WF_TTWU, it has never checked if sd_domain has SD_BALANCE_WAKE flag.
look at : #define WF_TTWU 0x08 /* Wakeup; maps to SD_BALANCE_WAKE */
so when wake_wide return false, we use the wake_affine mecanism but if it's false then we fllback to default mode which looks for: if (tmp->flags & sd_flag)
This means looking for SD_BALANCE_WAKE which is never set
so sd will stay NULL and you will end up calling select_idle_sibling anyway
static int select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags) { ...
if (wake_flags & WF_TTWU) { record_wakee(p); if (sched_energy_enabled()) { new_cpu = find_energy_efficient_cpu(p, prev_cpu); if (new_cpu >= 0) return new_cpu; new_cpu = prev_cpu; } want_affine = !wake_wide(p) && cpumask_test_cpu(cpu, p->cpus_ptr); }}
And try_to_wake_up() has always set WF_TTWU: static int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) { cpu = select_task_rq(p, p->wake_cpu, wake_flags | WF_TTWU); ... }
So the change in wake_wide will actually affect the value of want_affine. And I did also see code entered slow path during my benchmark.
Yes, this is happening but IMHO not for wakeups. Check wake_flags for the tasks which go through `slow path` on your machine. They should have WF_EXEC or WF_FORK, not WF_TTWU (& WF_SYNC).
One issue I mentioned during linaro open discussion is that since I have moved to use cluster size to decide the value of wake_wide, relatively less tasks will make wake_wide() decide to go to slow path, thus, tasks begin to spread to other NUMA, but actually llc_size might be able to contain those tasks. So a possible model might be: static int wake_wide(struct task_struct *p) { tasksize < cluster : scan cluster tasksize > llc : slow path tasksize > cluster && tasksize < llc: scan llc }
thoughts?
Like Vincent explained, the return value of wake_wide() doesn't matter. For wakeups you always end up in sis().
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