Messages in this thread Patch in this message | | | Subject | Re: [patch] x86, tlb: switch cr3 in leave_mm() only when needed | From | Suresh Siddha <> | Date | Thu, 22 Mar 2012 17:01:25 -0700 |
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On Thu, 2012-03-22 at 16:44 -0700, Linus Torvalds wrote: > 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?
Agreed. Updated patch appended. Thanks. --- From: Suresh Siddha <suresh.b.siddha@intel.com> Subject: x86, tlb: switch cr3 in leave_mm() only when needed
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.
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com> --- arch/x86/mm/tlb.c | 8 +++++--- 1 files changed, 5 insertions(+), 3 deletions(-)
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c index d6c0418..125bcad 100644 --- a/arch/x86/mm/tlb.c +++ b/arch/x86/mm/tlb.c @@ -61,11 +61,13 @@ static DEFINE_PER_CPU_READ_MOSTLY(int, tlb_vector_offset); */ void leave_mm(int cpu) { + struct mm_struct *active_mm = percpu_read(cpu_tlbstate.active_mm); if (percpu_read(cpu_tlbstate.state) == TLBSTATE_OK) BUG(); - cpumask_clear_cpu(cpu, - mm_cpumask(percpu_read(cpu_tlbstate.active_mm))); - load_cr3(swapper_pg_dir); + if (cpumask_test_cpu(cpu, mm_cpumask(active_mm))) { + cpumask_clear_cpu(cpu, mm_cpumask(active_mm)); + load_cr3(swapper_pg_dir); + } } EXPORT_SYMBOL_GPL(leave_mm);
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