Re: frequent lockups in 3.18rc4

[Linus Torvalds]

On Wed, Dec 3, 2014 at 7:15 PM, Chris Mason <clm@xxxxxx> 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?

                  Linus
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