From: Chris Wilson <chris@chris-wilson.co.uk>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: intel-gfx@lists.freedesktop.org,
David Laight <David.Laight@aculab.com>,
dri-devel@lists.freedesktop.org
Subject: Re: [Intel-gfx] [PATCH] drm: Inject a cond_resched() into long drm_clflush_sg()
Date: Thu, 16 Jan 2020 07:40:19 +0000 [thread overview]
Message-ID: <157916041994.14122.8524532515240369595@skylake-alporthouse-com> (raw)
In-Reply-To: <20200116065242.GC8400@dvetter-linux.ger.corp.intel.com>
Quoting Daniel Vetter (2020-01-16 06:52:42)
> On Wed, Jan 15, 2020 at 08:52:45PM +0000, Chris Wilson wrote:
> > Since we may try and flush the cachelines associated with large buffers
> > (an 8K framebuffer is about 128MiB, even before we try HDR), this leads
> > to unacceptably long latencies (when using a voluntary CONFIG_PREEMPT).
> > If we call cond_resched() between each sg chunk, that it about every 128
> > pages, we have a natural break point in which to check if the process
> > needs to be rescheduled. Naturally, this means that drm_clflush_sg() can
> > only be called from process context -- which is true at the moment. The
> > other clflush routines remain usable from atomic context.
> >
> > Even though flushing large objects takes a demonstrable amount to time
> > to flush all the cachelines, clflush is still preferred over a
> > system-wide wbinvd as the latter has unpredictable latencies affecting
> > the whole system not just the local task.
> >
> > Reported-by: David Laight <David.Laight@ACULAB.COM>
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: David Laight <David.Laight@ACULAB.COM>
>
> The original bug report is complaining about latencies for SCHED_RT
> threads, on a system that doesn't even use CONFIG_PREEMPT. I'm not sure
> it's terribly valid to cater to that use-case - all the desktop distros
> seem a lot more reasonable. So firmly *shrug* from my side ...
Yeah, I had the same immediate response to the complaint), but otoh we've
inserted cond_resched() before when it looks like may consume entire
jiffies inside a loop. At the very minimum, we should have a
might_sleep() here and a reminder that this can be very slow (remember
byt?).
-Chris
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next prev parent reply other threads:[~2020-01-16 7:40 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-15 20:52 [Intel-gfx] [PATCH] drm: Inject a cond_resched() into long drm_clflush_sg() Chris Wilson
2020-01-15 21:28 ` [Intel-gfx] ✗ Fi.CI.CHECKPATCH: warning for " Patchwork
2020-01-15 21:57 ` [Intel-gfx] ✓ Fi.CI.BAT: success " Patchwork
2020-01-15 21:57 ` [Intel-gfx] ✗ Fi.CI.BUILD: warning " Patchwork
2020-01-16 6:52 ` [Intel-gfx] [PATCH] " Daniel Vetter
2020-01-16 7:40 ` Chris Wilson [this message]
2020-01-16 12:26 ` David Laight
2020-01-16 12:28 ` Chris Wilson
2020-01-16 13:58 ` David Laight
2020-01-16 14:01 ` Chris Wilson
2020-01-16 14:40 ` David Laight
2020-01-16 15:04 ` David Laight
2020-01-18 10:45 ` [Intel-gfx] ✓ Fi.CI.IGT: success for " Patchwork
2020-01-23 14:36 ` [Intel-gfx] [PATCH] " David Laight
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