linux-kernel - schedule (wait

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Message-ID: <b170af451002270152u228a46abr3b316da75b2fa3f0@mail.gmail.com>
Date:	Sat, 27 Feb 2010 10:52:09 +0100
From:	Rafał Miłecki <zajec5@...il.com>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: schedule (wait_event) vs. while

In radeon engine reclocking code I need to do some calculations and
then wait for VBLANK interrupt. Right after VBLANK interrupt will
happen I need to reclock ASAP.

Interrupts are received by another context and I need some quite fast
synchronization.

1) Schedule solution:
reclocking() {
  calculations();
  radeon->vblank_sync = 0;
  wait_event_timeout(radeon->wq, radeon->vblank_sync, timeout);
  reclock();
}
i-handler() { radeon->vblank_sync = 1; }

2) While solution:
reclocking() {
  calculations();
  del_timer(radeon->fake_vblank);
  radeon->vblank_sync = 0;
  mod_timer(radeon->fake_vblank, 10ms);
  while(!radeon->vblank_sync);
  reclock();
}
i-handler() { radeon->vblank_sync = 1; }
fake-handler() { radeon->vblank_sync = 1; }

Of course first one is much cleaner but internally wait_event_timeout
uses schedule_timeout(). Now according to LDD3:
> Once a process releases the processor with schedule, there are no guarantees that the process will get the processor back anytime soon. Therefore, calling schedule in this manner is not a safe solution to the driver’s needs, in addition to being bad for the computing system as a whole. If you test jitsched while running load50, you can see that the delay associated to each line is extended by a few seconds, because other processes are using the CPU when the timeout expires.

I can not afford any delays because VBLANK is so short timeframe. Is
there any better solution?

-- 
Rafał
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