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

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Message-ID: <Z3hhyS-SodRQR4bU@slm.duckdns.org>
Date: Fri, 3 Jan 2025 12:16:41 -1000
From: Tejun Heo <tj@...nel.org>
To: Changwoo Min <multics69@...il.com>
Cc: void@...ifault.com, arighi@...dia.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

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:

The patch series generally look good to me. Peter, if things look okay to
you, I'll apply the series to sched_ext/for-6.14.

Thanks.

-- 
tejun