Messages in this thread |  | | Subject | Re: [v2 PATCH] mm: mmu_gather: remove __tlb_reset_range() for force flush | From | Yang Shi <> | Date | Mon, 13 May 2019 16:01:09 -0700 |
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On 5/13/19 9:38 AM, Will Deacon wrote: > On Fri, May 10, 2019 at 07:26:54AM +0800, Yang Shi wrote: >> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c >> index 99740e1..469492d 100644 >> --- a/mm/mmu_gather.c >> +++ b/mm/mmu_gather.c >> @@ -245,14 +245,39 @@ void tlb_finish_mmu(struct mmu_gather *tlb, >> { >> /* >> * If there are parallel threads are doing PTE changes on same range >> - * under non-exclusive lock(e.g., mmap_sem read-side) but defer TLB >> - * flush by batching, a thread has stable TLB entry can fail to flush >> - * the TLB by observing pte_none|!pte_dirty, for example so flush TLB >> - * forcefully if we detect parallel PTE batching threads. >> + * under non-exclusive lock (e.g., mmap_sem read-side) but defer TLB >> + * flush by batching, one thread may end up seeing inconsistent PTEs >> + * and result in having stale TLB entries. So flush TLB forcefully >> + * if we detect parallel PTE batching threads. >> + * >> + * However, some syscalls, e.g. munmap(), may free page tables, this >> + * needs force flush everything in the given range. Otherwise this >> + * may result in having stale TLB entries for some architectures, >> + * e.g. aarch64, that could specify flush what level TLB. >> */ >> - if (mm_tlb_flush_nested(tlb->mm)) { >> - __tlb_reset_range(tlb); >> - __tlb_adjust_range(tlb, start, end - start); >> + if (mm_tlb_flush_nested(tlb->mm) && !tlb->fullmm) { >> + /* >> + * Since we can't tell what we actually should have >> + * flushed, flush everything in the given range. >> + */ >> + tlb->freed_tables = 1; >> + tlb->cleared_ptes = 1; >> + tlb->cleared_pmds = 1; >> + tlb->cleared_puds = 1; >> + tlb->cleared_p4ds = 1; >> + >> + /* >> + * Some architectures, e.g. ARM, that have range invalidation >> + * and care about VM_EXEC for I-Cache invalidation, need force >> + * vma_exec set. >> + */ >> + tlb->vma_exec = 1; >> + >> + /* Force vma_huge clear to guarantee safer flush */ >> + tlb->vma_huge = 0; >> + >> + tlb->start = start; >> + tlb->end = end; >> } > Whilst I think this is correct, it would be interesting to see whether > or not it's actually faster than just nuking the whole mm, as I mentioned > before. > > At least in terms of getting a short-term fix, I'd prefer the diff below > if it's not measurably worse.
I did a quick test with ebizzy (96 threads with 5 iterations) on my x86 VM, it shows slightly slowdown on records/s but much more sys time spent with fullmm flush, the below is the data.
nofullmm fullmm ops (records/s) 225606 225119 sys (s) 0.69 1.14
It looks the slight reduction of records/s is caused by the increase of sys time.
> > Will > > --->8 > > diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c > index 99740e1dd273..cc251422d307 100644 > --- a/mm/mmu_gather.c > +++ b/mm/mmu_gather.c > @@ -251,8 +251,9 @@ void tlb_finish_mmu(struct mmu_gather *tlb, > * forcefully if we detect parallel PTE batching threads. > */ > if (mm_tlb_flush_nested(tlb->mm)) { > + tlb->fullmm = 1; > __tlb_reset_range(tlb); > - __tlb_adjust_range(tlb, start, end - start); > + tlb->freed_tables = 1; > } > > tlb_flush_mmu(tlb);
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