Messages in this thread | | | Subject | Re: x86: clean up smpboot.c's use of udelay+schedule | From | Peter Zijlstra <> | Date | Tue, 31 Jan 2012 13:47:56 +0100 |
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On Tue, 2012-01-31 at 13:43 +0100, Ingo Molnar wrote: > * Arjan van de Ven <arjan@infradead.org> wrote: > > > >From 9f8dd2b15ff19ad73ee0eb235b4fdde9277185e8 Mon Sep 17 00:00:00 2001 > > From: Arjan van de Ven <arjan@linux.intel.com> > > Date: Mon, 30 Jan 2012 20:38:09 -0800 > > Subject: [PATCH] x86: clean up smpboot.c's use of udelay+schedule > > > > smpboot.c does a udelay() followed by a schedule(); to yield > > the CPU to other threads. This comes from the time when the kernel > > did not yet have usleep_*() style APIs... > > > > ... nowadays, the kernel can do better than this, and this > > patch replaces this code sequence with a usleep_range(), > > so that the CPU is actually yielded for some real time. > > > > Signed-off-by: Arjan van de Ven <arjan@linux.intel.com> > > --- > > arch/x86/kernel/smpboot.c | 3 +-- > > 1 files changed, 1 insertions(+), 2 deletions(-) > > > > diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c > > index 66d250c..0b794c6 100644 > > --- a/arch/x86/kernel/smpboot.c > > +++ b/arch/x86/kernel/smpboot.c > > @@ -781,14 +781,13 @@ do_rest: > > for (timeout = 0; timeout < 50000; timeout++) { > > if (cpumask_test_cpu(cpu, cpu_callin_mask)) > > break; /* It has booted */ > > - udelay(100); > > /* > > * Allow other tasks to run while we wait for the > > * AP to come online. This also gives a chance > > * for the MTRR work(triggered by the AP coming online) > > * to be completed in the stop machine context. > > */ > > - schedule(); > > + usleep_range(100, 200); > > I'm wondering whether we could shorten this delay to say 10 > usecs and thus save 0.1 msecs (or more) from a typical SMP > bootup?
wait_on_completion_timeout() and have the fresh cpu do a wakeup when its done. That way there's no need for a minimal wait time.
Anyway, all the cpu hotplug code is a friggin trainwreck and needs a complete rewrite across all archs.
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