RE: [PATCH v2 1/2] iopoll: Call cpu_relax() in busy loops

[David Laight <David.Laight@ACULAB.COM>]

From: Geert Uytterhoeven
> Sent: 23 May 2023 08:30
> 
> Hi David,
> 
> On Thu, May 11, 2023 at 12:49 PM David Laight <David.Laight@aculab.com> wrote:
> > > * Geert Uytterhoeven <geert+renesas@glider.be> [230510 13:23]:
> > > > It is considered good practice to call cpu_relax() in busy loops, see
> > > > Documentation/process/volatile-considered-harmful.rst.  This can not
> > > > only lower CPU power consumption or yield to a hyperthreaded twin
> > > > processor, but also allows an architecture to mitigate hardware issues
> > > > (e.g. ARM Erratum 754327 for Cortex-A9 prior to r2p0) in the
> > > > architecture-specific cpu_relax() implementation.
> >
> > Don't you also need to call cond_resched() (at least some times).
> > Otherwise the process can't be pre-empted and a RT process
> > that last ran on that cpu will never be scheduled.
> 
> According to [1], cond_resched() must be called at least once per few
> tens of milliseconds.

Hmmm.... tens of milliseconds is really much too long for RT threads.
A limit nearer 1ms would be barely acceptable.

	David

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