Messages in this thread | | | Date | Thu, 04 Dec 2014 07:22:25 -0800 | From | Dave Hansen <> | Subject | Re: frequent lockups in 3.18rc4 |
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On 12/03/2014 09:49 PM, Linus Torvalds wrote: > On Wed, Dec 3, 2014 at 7:15 PM, Chris Mason <clm@fb.com> wrote: >> >> One guess is that trinity is generating a huge number of tlb >> invalidations over sparse and horrible ranges. Perhaps the old code was >> falling back to full tlb flushes before Dave Hansen's string of fixes? > > Hmm. I agree that we've had some of the backtraces look like TLB > flushing might be involved. Not all, though. And I'm not seeing where > a loop over up to 33 pages should matter over doing a full TLB flush. > > What *might* matter is if we somehow get that number wrong, and the loops like > > addr = f->flush_start; > while (addr < f->flush_end) { > __flush_tlb_single(addr); > addr += PAGE_SIZE; > } > > ends up looping a *lot* due to some bug, and then the IPI itself would > take so long that the watchdog could trigger. > > But I do not see how that could actually happen. As far as I can tell, > either the number of pages is limited to less than 33, or we have that > TLB_FLUSH_ALL case. > > Do you see something I don't?
The one thing I _do_ see now is a missed TLB flush is we're flushing one page at the end of the address space. We'd overflow flush_end back so flush_end=0:
if (!f->flush_end) f->flush_end = f->flush_start + PAGE_SIZE; <-- overflow
and we'll never enter the while loop where we actually do the flush:
while (addr < f->flush_end) { __flush_tlb_single(addr); addr += PAGE_SIZE; }
But we have a hole up there on x86_64, so this will never happen in practice there. It might theoretically apply to 32-bit, but this still doesn't help with the bug.
Oh, and the tracepoint is spitting out bogus numbers because we need some parenthesis around the 'nr_pages' calculation.
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