Re: [PATCH] drm/nouveau: fix __nouveau_fence_wait performance regression

[Marcin Slusarz <marcin.slusarz-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On Tue, Mar 08, 2011 at 01:58:50AM +0100, Francisco Jerez wrote:
> Marcin Slusarz <marcin.slusarz-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> writes:
> 
> > On Tue, Mar 08, 2011 at 08:24:26AM +1000, Ben Skeggs wrote:
> >> On Mon, 2011-03-07 at 18:18 +0000, Maarten Maathuis wrote:
> >> > On Fri, Mar 4, 2011 at 4:49 PM, Marcin Slusarz <marcin.slusarz-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> >> > > On Sun, Feb 13, 2011 at 09:38:04PM +0100, Marcin Slusarz wrote:
> >> > >> Combination of locking and interchannel synchronization changes
> >> > >> uncovered poor behaviour of nouveau_fence_wait, which on HZ=100
> >> > >> configuration could waste up to 10 ms per call.
> >> > >> Depending on application, it lead to 10-30% FPS regression.
> >> > >> To fix it, shorten thread sleep time to 0.1 ms and ensure
> >> > >> spinning happens for at least one *full* tick.
> >> > >>
> >> > >> Signed-off-by: Marcin Slusarz <marcin.slusarz-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> >> > >> ---
> >> > >>  drivers/gpu/drm/nouveau/nouveau_fence.c |   10 ++++++++--
> >> > >>  1 files changed, 8 insertions(+), 2 deletions(-)
> >> > >>
> >> > >> diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.c b/drivers/gpu/drm/nouveau/nouveau_fence.c
> >> > >> index 221b846..75ba5e2 100644
> >> > >> --- a/drivers/gpu/drm/nouveau/nouveau_fence.c
> >> > >> +++ b/drivers/gpu/drm/nouveau/nouveau_fence.c
> >> > >> @@ -27,6 +27,9 @@
> >> > >>  #include "drmP.h"
> >> > >>  #include "drm.h"
> >> > >>
> >> > >> +#include <linux/ktime.h>
> >> > >> +#include <linux/hrtimer.h>
> >> > >> +
> >> > >>  #include "nouveau_drv.h"
> >> > >>  #include "nouveau_ramht.h"
> >> > >>  #include "nouveau_dma.h"
> >> > >> @@ -230,9 +233,12 @@ int
> >> > >>  __nouveau_fence_wait(void *sync_obj, void *sync_arg, bool lazy, bool intr)
> >> > >>  {
> >> > >>       unsigned long timeout = jiffies + (3 * DRM_HZ);
> >> > >> -     unsigned long sleep_time = jiffies + 1;
> >> > >> +     unsigned long sleep_time = jiffies + 2;
> >> > >> +     ktime_t t;
> >> > >>       int ret = 0;
> >> > >>
> >> > >> +     t = ktime_set(0, NSEC_PER_MSEC / 10);
> >> > >> +
> >> > >>       while (1) {
> >> > >>               if (__nouveau_fence_signalled(sync_obj, sync_arg))
> >> > >>                       break;
> >> > >> @@ -245,7 +251,7 @@ __nouveau_fence_wait(void *sync_obj, void *sync_arg, bool lazy, bool intr)
> >> > >>               __set_current_state(intr ? TASK_INTERRUPTIBLE
> >> > >>                       : TASK_UNINTERRUPTIBLE);
> >> > >>               if (lazy && time_after_eq(jiffies, sleep_time))
> >> > >> -                     schedule_timeout(1);
> >> > >> +                     schedule_hrtimeout(&t, HRTIMER_MODE_REL);
> >> > >>
> >> > >>               if (intr && signal_pending(current)) {
> >> > >>                       ret = -ERESTARTSYS;
> >> > >> --
> >> > >> 1.7.4.rc3
> >> > >>
> >> > >
> >> > > ping again
> >> > > _______________________________________________
> >> > > Nouveau mailing list
> >> > > Nouveau-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org
> >> > > http://lists.freedesktop.org/mailman/listinfo/nouveau
> >> > >
> >> > 
> >> > This looks ok to me, but I would like to get Ben Skeggs ok on this one
> >> > as well. So i've CC'ed him, hopefully he'll notice :-)
> >> Ah sorry, I have actually looked at this quite a while back but came to
> >> no solid conclusion.
> >> 
> >> While yes, I did see some minor performance improvement from it, I also
> >> notice that now we once again get 100% CPU usage while an app is waiting
> >> for the GPU a lot..
> >
> > It's not "minor" performance improvement:
> >
> > without this patch (FPS):
> > nexuiz:    53
> > wop:       181
> > tremulous: 157
> > wsw0.5:    89
> > glxgears:  730
> >
> > with:
> > nexuiz:    63   (+18%)
> > wop:       248  (+37%)
> > tremulous: 156  (-0.6%)
> > wsw0.5:    91   (+2%)
> > glxgears:  1054 (+44%)
> >
> >
> > Ok, so you are worried about CPU usage... Let's see what will happen if
> > I remove spinning added by "drm/nouveau: Spin for a bit in 
> > nouveau_fence_wait() before yielding the CPU".
> >
> > reduced version (attached):
> > nexuiz:    62
> > wop:       248
> > trem:      157
> > wsw0.5:    90
> > glxgears:  1055
> >
> > Good enough?
> 
> Remember to exercise some software fallbacks as well (e.g. something
> using core fonts), software fallbacks were the main users of the
> spinning you've removed.

corefonts are pretty fast (measured "time dmesg"):

without (spinning + timeout 10ms): 0.08s
with (spinning + hrtimeout 0.1ms): 0.08s
reduced (no spinning + hrtimeout 0.1ms): 0.25s
old (no spinning + timeout 10ms): 13s

So I think "no spinning + hrtimeout 0.1ms" is a reasonable compromise...

BTW, old behaviour (no spinning + timeout 10ms) affects other workloads too
nexuiz:      50
wop:        153
tremulous:  155
wsw0.5:      89
glxgears:   100 (!)

> Anyway, software fallbacks and occlusion queries are the only two places
> (that I can think of now) where we need the low latency your patch
> gives, and, as Ben already pointed out, we probably want to keep CPU
> usage at minimum in every other case.  As a middle ground, the "lazy"
> flag (or rather, a "hog" flag?) could be exposed all the way up to
> userspace, and those two cases fixed to set the flag differently.
> 
> What do you think?

I'm not sure. I think optimizing for low CPU usage is not the best what
we can do right now. 3D performance is still too low behind blob.
Let's fix 3D perf first and think about CPU usage later.

> >
> > ---
> > From: Marcin Slusarz <marcin.slusarz-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> > Subject: [PATCH] drm/nouveau: fix __nouveau_fence_wait performance regression
> >
> > Combination of locking and interchannel synchronization changes
> > uncovered poor behaviour of nouveau_fence_wait, which on HZ=100
> > configuration could waste up to 10 ms per call.
> > Depending on application, it lead to 10-30% FPS regression.
> >
> > To fix it, shorten thread sleep time to 0.1 ms.
> >
> > Additionally, remove spinning (added by "drm/nouveau: Spin for
> > a bit in nouveau_fence_wait() before yielding the CPU"), because
> > it's not needed anymore.
> >
> > Signed-off-by: Marcin Slusarz <marcin.slusarz-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> > ---
> >  drivers/gpu/drm/nouveau/nouveau_fence.c |   11 ++++++++---
> >  1 files changed, 8 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.c b/drivers/gpu/drm/nouveau/nouveau_fence.c
> > index a244702..010243b 100644
> > --- a/drivers/gpu/drm/nouveau/nouveau_fence.c
> > +++ b/drivers/gpu/drm/nouveau/nouveau_fence.c
> > @@ -27,6 +27,9 @@
> >  #include "drmP.h"
> >  #include "drm.h"
> >  
> > +#include <linux/ktime.h>
> > +#include <linux/hrtimer.h>
> > +
> >  #include "nouveau_drv.h"
> >  #include "nouveau_ramht.h"
> >  #include "nouveau_dma.h"
> > @@ -229,9 +232,11 @@ int
> >  __nouveau_fence_wait(void *sync_obj, void *sync_arg, bool lazy, bool intr)
> >  {
> >  	unsigned long timeout = jiffies + (3 * DRM_HZ);
> > -	unsigned long sleep_time = jiffies + 1;
> > +	ktime_t t;
> >  	int ret = 0;
> >  
> > +	t = ktime_set(0, NSEC_PER_MSEC / 10);
> > +
> >  	while (1) {
> >  		if (__nouveau_fence_signalled(sync_obj, sync_arg))
> >  			break;
> > @@ -243,8 +248,8 @@ __nouveau_fence_wait(void *sync_obj, void *sync_arg, bool lazy, bool intr)
> >  
> >  		__set_current_state(intr ? TASK_INTERRUPTIBLE
> >  			: TASK_UNINTERRUPTIBLE);
> > -		if (lazy && time_after_eq(jiffies, sleep_time))
> > -			schedule_timeout(1);
> > +		if (lazy)
> > +			schedule_hrtimeout(&t, HRTIMER_MODE_REL);
> >  
> >  		if (intr && signal_pending(current)) {
> >  			ret = -ERESTARTSYS;