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Message-ID: <CA+55aFyf9RqAROx7t=CYUSGCDBXvv9z2c+LP1vRJjLU45_pD+Q@mail.gmail.com>
Date: Thu, 22 Mar 2012 16:44:54 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Suresh Siddha <suresh.b.siddha@...el.com>
Cc: Ingo Molnar <mingo@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
Len Brown <lenb@...nel.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [patch] x86, tlb: switch cr3 in leave_mm() only when needed
On Thu, Mar 22, 2012 at 4:33 PM, Suresh Siddha
<suresh.b.siddha@...el.com> wrote:
>
> Currently leave_mm() unconditionally switches the cr3 to swapper_pg_dir.
> But there is no need to change the cr3, if we already left that mm.
>
> intel_idle() for example calls leave_mm() on every deep c-state entry where
> the CPU flushes the TLB for us. Similarly flush_tlb_all() was also calling
> leave_mm() whenever the TLB is in LAZY state. Both these paths will be
> improved with this change.
Hmm. If this is reasonably common (and intel_idle() certainly is),
maybe we shouldn't even do the "test_and_clear" RMW cycle.
We could do it with a read-only bit test (no races I can see - if it's
clear, it will stay clear), so we could do this with
if (cpumask_test_cpu(cpu, mm_cpumask(active_mm))) {
cpumask_clear_cpu(cpu,mm_cpumask(active_mm));
load_cr3(swapper_pg_dir);
}
instead? And avoid touching that "mm_cpumask" (and the atomic
serializing instruction) when not necessary?
Linus
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