On Thu, Dec 4, 2014 at 10:22 AM, Dave Hansen <dave.han...@intel.com>
wrote:
On 12/03/2014 09:49 PM, Linus Torvalds wrote:
On Wed, Dec 3, 2014 at 7:15 PM, Chris Mason <c...@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
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