Messages in this thread | | | Date | Mon, 30 May 2022 23:26:09 +0100 | From | "Russell King (Oracle)" <> | Subject | Re: [PATCH v5] mm: Avoid unnecessary page fault retires on shared memory types |
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On Mon, May 30, 2022 at 02:34:50PM -0400, Peter Xu wrote: > I observed that for each of the shared file-backed page faults, we're very > likely to retry one more time for the 1st write fault upon no page. It's > because we'll need to release the mmap lock for dirty rate limit purpose > with balance_dirty_pages_ratelimited() (in fault_dirty_shared_page()). > > Then after that throttling we return VM_FAULT_RETRY. > > We did that probably because VM_FAULT_RETRY is the only way we can return > to the fault handler at that time telling it we've released the mmap lock. > > However that's not ideal because it's very likely the fault does not need > to be retried at all since the pgtable was well installed before the > throttling, so the next continuous fault (including taking mmap read lock, > walk the pgtable, etc.) could be in most cases unnecessary. > > It's not only slowing down page faults for shared file-backed, but also add > more mmap lock contention which is in most cases not needed at all. > > To observe this, one could try to write to some shmem page and look at > "pgfault" value in /proc/vmstat, then we should expect 2 counts for each > shmem write simply because we retried, and vm event "pgfault" will capture > that. > > To make it more efficient, add a new VM_FAULT_COMPLETED return code just to > show that we've completed the whole fault and released the lock. It's also > a hint that we should very possibly not need another fault immediately on > this page because we've just completed it. > > This patch provides a ~12% perf boost on my aarch64 test VM with a simple > program sequentially dirtying 400MB shmem file being mmap()ed and these are > the time it needs: > > Before: 650.980 ms (+-1.94%) > After: 569.396 ms (+-1.38%) > > I believe it could help more than that. > > We need some special care on GUP and the s390 pgfault handler (for gmap > code before returning from pgfault), the rest changes in the page fault > handlers should be relatively straightforward. > > Another thing to mention is that mm_account_fault() does take this new > fault as a generic fault to be accounted, unlike VM_FAULT_RETRY. > > I explicitly didn't touch hmm_vma_fault() and break_ksm() because they do > not handle VM_FAULT_RETRY even with existing code, so I'm literally keeping > them as-is. > > Acked-by: Geert Uytterhoeven <geert@linux-m68k.org> > Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> > Acked-by: Johannes Weiner <hannes@cmpxchg.org> > Acked-by: Vineet Gupta <vgupta@kernel.org> > Acked-by: Guo Ren <guoren@kernel.org> > Acked-by: Max Filippov <jcmvbkbc@gmail.com> > Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com> > Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc) > Acked-by: Catalin Marinas <catalin.marinas@arm.com> > Reviewed-by: Alistair Popple <apopple@nvidia.com> > Reviewed-by: Ingo Molnar <mingo@kernel.org> > Signed-off-by: Peter Xu <peterx@redhat.com>
For:
> diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c > index a062e07516dd..46cccd6bf705 100644 > --- a/arch/arm/mm/fault.c > +++ b/arch/arm/mm/fault.c > @@ -322,6 +322,10 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs) > return 0; > } > > + /* The fault is fully completed (including releasing mmap lock) */ > + if (fault & VM_FAULT_COMPLETED) > + return 0; > + > if (!(fault & VM_FAULT_ERROR)) { > if (fault & VM_FAULT_RETRY) { > flags |= FAULT_FLAG_TRIED;
Acked-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Thanks!
-- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!
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