linux-kernel - Re: [PATCH v7 0/6] sched_ext: Support high-performance monotonically non-decreasing clock

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Message-ID: <Z3bg5Jerg8c1SVWq@gpd3>
Date: Thu, 2 Jan 2025 19:54:28 +0100
From: Andrea Righi <arighi@...dia.com>
To: Changwoo Min <multics69@...il.com>
Cc: tj@...nel.org, void@...ifault.com, mingo@...hat.com,
	peterz@...radead.org, changwoo@...lia.com, kernel-dev@...lia.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 0/6] sched_ext: Support high-performance monotonically
 non-decreasing clock

Hi Changwoo,

On Mon, Dec 30, 2024 at 06:56:19PM +0900, Changwoo Min wrote:
> Many BPF schedulers (such as scx_central, scx_lavd, scx_rusty, scx_bpfland,
> and scx_flash) frequently call bpf_ktime_get_ns() for tracking tasks' runtime
> properties. If supported, bpf_ktime_get_ns() eventually reads a hardware
> timestamp counter (TSC). However, reading a hardware TSC is not
> performant in some hardware platforms, degrading IPC.
> 
> This patchset addresses the performance problem of reading hardware TSC
> by leveraging the rq clock in the scheduler core, introducing a
> scx_bpf_now() function for BPF schedulers. Whenever the rq clock
> is fresh and valid, scx_bpf_now() provides the rq clock, which is
> already updated by the scheduler core (update_rq_clock), so it can reduce
> reading the hardware TSC.
> 
> When the rq lock is released (rq_unpin_lock), the rq clock is invalidated,
> so a subsequent scx_bpf_now() call gets the fresh sched_clock for the caller.
> 
> In addition, scx_bpf_now() guarantees the clock is monotonically
> non-decreasing for the same CPU, so the clock cannot go backward
> in the same CPU.
> 
> Using scx_bpf_now() reduces the number of reading hardware TSC
> by 50-80% (76% for scx_lavd, 82% for scx_bpfland, and 51% for scx_rusty)
> for the following benchmark:
> 
>     perf bench -f simple sched messaging -t -g 20 -l 6000

Looks good to me, I also tested it and I have nothing to report, therefore:

Acked-by: Andrea Righi <arighi@...dia.com>

Thanks,
-Andrea