Re: sandy bridge kswapd0 livelock with pagecache

[Mel Gorman <mgorman@suse.de>]

On Tue, Jun 21, 2011 at 12:59:00PM +0100, P?draig Brady wrote:
> On 21/06/11 12:34, Mel Gorman wrote:
> > On Tue, Jun 21, 2011 at 11:47:35AM +0100, P?draig Brady wrote:
> >> On 21/06/11 11:39, Mel Gorman wrote:
> >>> On Tue, Jun 21, 2011 at 10:53:02AM +0100, P?draig Brady wrote:
> >>>> I tried the 2 patches here to no avail:
> >>>> http://marc.info/?l=linux-mm&m=130503811704830&w=2
> >>>>
> >>>> I originally logged this at:
> >>>> https://bugzilla.redhat.com/show_bug.cgi?id=712019
> >>>>
> >>>> I can compile up and quickly test any suggestions.
> >>>>
> >>>
> >>> I recently looked through what kswapd does and there are a number
> >>> of problem areas. Unfortunately, I haven't gotten around to doing
> >>> anything about it yet or running the test cases to see if they are
> >>> really problems. In your case, the following is a strong possibility
> >>> though. This should be applied on top of the two patches merged from
> >>> that thread.
> >>>
> >>> This is not tested in any way, based on 3.0-rc3
> >>
> >> This does not fix the issue here.
> >>
> > 
> > I made a silly mistake here.  When you mentioned two patches applied,
> > I assumed you meant two patches that were finally merged from that
> > discussion thread instead of looking at your linked mail. Now that I
> > have checked, I think you applied the SLUB patches while the patches
> > I was thinking of are;
> > 
> > [afc7e326: mm: vmscan: correct use of pgdat_balanced in sleeping_prematurely]
> > [f06590bd: mm: vmscan: correctly check if reclaimer should schedule during shrink_slab]
> > 
> > The first one in particular has been reported by another user to fix
> > hangs related to copying large files. I'm assuming you are testing
> > against the Fedora kernel. As these patches were merged for 3.0-rc1, can
> > you check if applying just these two patches to your kernel helps?
> 
> These patches are already present in my 2.6.38.8-32.fc15.x86_64 kernel :(
> 

Would it be possible to record a profile while it is livelocked to check
if it's stuck in this loop in shrink_slab()?

                while (total_scan >= SHRINK_BATCH) {
                        long this_scan = SHRINK_BATCH;
                        int shrink_ret;
                        int nr_before;

                        nr_before = do_shrinker_shrink(shrinker, shrink, 0);
                        shrink_ret = do_shrinker_shrink(shrinker, shrink,
                                                        this_scan);
                        if (shrink_ret == -1)
                                break;
                        if (shrink_ret < nr_before)
                                ret += nr_before - shrink_ret;
                        count_vm_events(SLABS_SCANNED, this_scan);
                        total_scan -= this_scan;

                        cond_resched();
                }

Also, can you post the output of sysrq+m at a few different times while
kswapd is spinning heavily? I want to see if all_unreclaimable has been
set on zones with a reasonable amount of memory. If they are, it's
possible for kswapd to be in a continual loop calling shrink_slab() and
skipping over normal page reclaim because all_unreclaimable is set
everywhere until a page is freed.

-- 
Mel Gorman
SUSE Labs

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