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

[Peter Zijlstra]

On Tue, Jan 07, 2025 at 09:46:48AM -1000, Tejun Heo wrote:
> Hello,
> 
> On Fri, Jan 03, 2025 at 12:16:41PM -1000, Tejun Heo wrote:
> > 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.
> 
> Applying to sched_ext/for-6.14. Please holler if there are concerns.

Urgh, I'll try and have a look, but have a hate for everybody who's been
working through x-mas :/