On 2/9/21 6:46 PM, Yang Shi wrote: > The number of deferred objects might get windup to an absurd number, and it > results in clamp of slab objects. It is undesirable for sustaining workingset. > > So shrink deferred objects proportional to priority and cap nr_deferred to twice > of cache items. Makes sense to me, minimally it's simpler than the old code and avoiding absurd growth of nr_deferred should be a good thing, as well as the "proportional to priority" part. I just suspect there's a bit of unnecessary bias in the implementation, as explained below: > The idea is borrowed from Dave Chinner's patch: > https://lore.kernel.org/linux-xfs/20191031234618.15403-13-david@xxxxxxxxxxxxx/ > > Tested with kernel build and vfs metadata heavy workload in our production > environment, no regression is spotted so far. > > Signed-off-by: Yang Shi <shy828301@xxxxxxxxx> > --- > mm/vmscan.c | 40 +++++----------------------------------- > 1 file changed, 5 insertions(+), 35 deletions(-) > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 66163082cc6f..d670b119d6bd 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -654,7 +654,6 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl, > */ > nr = count_nr_deferred(shrinker, shrinkctl); > > - total_scan = nr; > if (shrinker->seeks) { > delta = freeable >> priority; > delta *= 4; > @@ -668,37 +667,9 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl, > delta = freeable / 2; > } > > + total_scan = nr >> priority; > total_scan += delta; So, our scan goal consists of the part based on freeable objects (delta), plus a part of the defferred objects (nr >> priority). Fine. > - if (total_scan < 0) { > - pr_err("shrink_slab: %pS negative objects to delete nr=%ld\n", > - shrinker->scan_objects, total_scan); > - total_scan = freeable; > - next_deferred = nr; > - } else > - next_deferred = total_scan; > - > - /* > - * We need to avoid excessive windup on filesystem shrinkers > - * due to large numbers of GFP_NOFS allocations causing the > - * shrinkers to return -1 all the time. This results in a large > - * nr being built up so when a shrink that can do some work > - * comes along it empties the entire cache due to nr >>> > - * freeable. This is bad for sustaining a working set in > - * memory. > - * > - * Hence only allow the shrinker to scan the entire cache when > - * a large delta change is calculated directly. > - */ > - if (delta < freeable / 4) > - total_scan = min(total_scan, freeable / 2); > - > - /* > - * Avoid risking looping forever due to too large nr value: > - * never try to free more than twice the estimate number of > - * freeable entries. > - */ > - if (total_scan > freeable * 2) > - total_scan = freeable * 2; > + total_scan = min(total_scan, (2 * freeable)); Probably unnecessary as we cap next_deferred below anyway? So total_scan cannot grow without limits anymore. But can't hurt. > trace_mm_shrink_slab_start(shrinker, shrinkctl, nr, > freeable, delta, total_scan, priority); > @@ -737,10 +708,9 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl, > cond_resched(); > } > > - if (next_deferred >= scanned) > - next_deferred -= scanned; > - else > - next_deferred = 0; > + next_deferred = max_t(long, (nr - scanned), 0) + total_scan; And here's the bias I think. Suppose we scanned 0 due to e.g. GFP_NOFS. We count as newly deferred both the "delta" part of total_scan, which is fine, but also the "nr >> priority" part, where we failed to our share of the "reduce nr_deferred" work, but I don't think it means we should also increase nr_deferred by that amount of failed work. OTOH if we succeed and scan exactly the whole goal, we are subtracting from nr_deferred both the "nr >> priority" part, which is correct, but also delta, which was new work, not deferred one, so that's incorrect IMHO as well. So the calculation should probably be something like this? next_deferred = max_t(long, nr + delta - scanned, 0); Thanks, Vlastimil > + next_deferred = min(next_deferred, (2 * freeable)); > + > /* > * move the unused scan count back into the shrinker in a > * manner that handles concurrent updates. >
