Re: [RFC PATCH] sched: Queue task on wakelist in the same llc if the wakee cpu is idle

[Peter Zijlstra <peterz@infradead.org>]

On Fri, May 13, 2022 at 02:24:27PM +0800, Tianchen Ding wrote:
> We notice the commit 518cd6234178 ("sched: Only queue remote wakeups
> when crossing cache boundaries") disabled queuing tasks on wakelist when
> the cpus share llc. This is because, at that time, the scheduler must
> send IPIs to do ttwu_queue_wakelist.

No; this was because of cache bouncing.

> Nowadays, ttwu_queue_wakelist also
> supports TIF_POLLING, so this is not a problem now when the wakee cpu is
> in idle polling.
> 
> Benefits:
>   Queuing the task on idle cpu can help improving performance on waker cpu
>   and utilization on wakee cpu, and further improve locality because
>   the wakee cpu can handle its own rq. This patch helps improving rt on
>   our real java workloads where wakeup happens frequently.
> 
> Does this patch bring IPI flooding?
>   For archs with TIF_POLLING_NRFLAG (e.g., x86), there will be no
>   difference if the wakee cpu is idle polling. If the wakee cpu is idle
>   but not polling, the later check_preempt_curr() will send IPI too.
> 
>   For archs without TIF_POLLING_NRFLAG (e.g., arm64), the IPI is
>   unavoidable, since the later check_preempt_curr() will send IPI when
>   wakee cpu is idle.
> 
> Benchmark:
> running schbench -m 2 -t 8 on 8269CY:
> 
> without patch:
> Latency percentiles (usec)
>         50.0000th: 10
>         75.0000th: 14
>         90.0000th: 16
>         95.0000th: 16
>         *99.0000th: 17
>         99.5000th: 20
>         99.9000th: 23
>         min=0, max=28
> 
> with patch:
> Latency percentiles (usec)
>         50.0000th: 6
>         75.0000th: 8
>         90.0000th: 9
>         95.0000th: 9
>         *99.0000th: 10
>         99.5000th: 10
>         99.9000th: 14
>         min=0, max=16
> 
> We've also tested unixbench and see about 10% improvement on Pipe-based
> Context Switching, and no performance regression on other test cases.
> 
> For arm64, we've tested schbench and unixbench on Kunpeng920, the
> results show that,

What is a kunpeng and how does it's topology look?

> the improvement is not as obvious as on x86, and
> there's no performance regression.

x86 is wide and varied; what x86 did you test?