From: Rik van Riel <riel@surriel.com>
To: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>,
Andi Kleen <andi@firstfloor.org>,
linux-kernel <linux-kernel@vger.kernel.org>
Subject: question about lazy FPU restore
Date: Thu, 02 May 2013 18:19:19 -0400 [thread overview]
Message-ID: <5182E667.1000603@surriel.com> (raw)
Hi Suresh,
I have a question about the lazy fpu restore code in
switch_fpu_prepare. Specifically, about the case where
the old task did not use the FPU, and the new task's
FPU state is still in the cpu.
} else {
old->fpu_counter = 0;
old->thread.fpu.last_cpu = ~0;
if (fpu.preload) {
new->fpu_counter++;
if (!use_eager_fpu() && fpu_lazy_restore(new, cpu))
fpu.preload = 0;
else
prefetch(new->thread.fpu.state);
__thread_fpu_begin(new);
}
}
In this branch, we call fpu_lazy_restore, which
confirms that the CPU still has the new task's state
in it.
However, if we are in eager fpu mode, we still end up
calling restore_fpu_checking from switch_fpu_finish,
even if the new task's FPU state is still resident in
the CPU.
Is there a particular reason we do this?
Would it be possible to always set fpu.preload = 0,
call clts, and __thread_set_has_fpu if fpu_lazy_restore
returns true?
That would allow us to skip the loading of FPU state
when re-entering a process that went briefly idle, before
getting something else to do, a common occurrance in
message passing workloads.
reply other threads:[~2013-05-02 22:21 UTC|newest]
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