Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH v2 1/2] iopoll: Call cpu_relax() in busy loops | Date | Tue, 23 May 2023 08:55:08 +0000 |
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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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