Messages in this thread | | | Date | Thu, 4 Dec 2014 10:30:38 -0500 | From | Chris Mason <> | Subject | Re: frequent lockups in 3.18rc4 |
| |
On Thu, Dec 4, 2014 at 10:22 AM, Dave Hansen <dave.hansen@intel.com> wrote: > 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.
Yeah, I didn't see any problems with your changes, but I was hoping that even a small change like doing 33 flushes at a time was pushing Dave's box just over the line.
-chris
| |