Messages in this thread | | | From | Linus Torvalds <> | Date | Thu, 22 Mar 2012 16:44:54 -0700 | Subject | Re: [patch] x86, tlb: switch cr3 in leave_mm() only when needed |
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On Thu, Mar 22, 2012 at 4:33 PM, Suresh Siddha <suresh.b.siddha@intel.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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