Re: [PATCH] mm/lruvec: preemptively free dead folios during lru_add drain

[Shakeel Butt]

On Thu, Apr 23, 2026 at 09:43:07AM -0700, JP Kobryn (Meta) wrote:
> Of all observable lruvec lock contention in our fleet, we find that ~24%
> occurs when dead folios are present in lru_add batches at drain time.

So, when they were added to the percpu lru cache, they were alive but during
their stay in lru cache, they were freed (last non-lrucache ref dropped) or
somehow we are adding folio where the caller drops the reference just after
adding to percpu lru cache e.g. folio_putback_lru() ?

> This
> is wasteful in the sense that the folio is added to the LRU just to be
> immediately removed via folios_put_refs(), incurring two unnecessary lock
> acquisitions.
> 
> Eliminate this overhead by preemptively cleaning up dead folios before they
> make it into the LRU. Use folio_ref_freeze() to filter folios whose only
> remaining refcount is the batch ref. When dead folios are found, move them
> off the add batch and onto a temporary batch to be freed.
> 
> During A/B testing on one of our prod instagram workloads (high-frequency
> short-lived requests), the patch intercepted almost all dead folios before
> they entered the LRU. Data collected using the mm_lru_insertion tracepoint
> shows the effectiveness of the patch:
> 
> Per-host LRU add averages at 95% CPU load
> (60 hosts each side, 3 x 60s intervals)
> 
>             dead folios/min  total folios/min   dead %
> unpatched:        1,297,785        19,341,986  6.7097%
> patched:                 14        19,039,996  0.0001%
> 
> Within this workload, we save ~2.6M lock acquisitions per minute per host
> as a result.
> 
> System-wide memory stats improved on the patched side also at 95% CPU load:
>  - direct reclaim scanning reduced 7%
>  - allocation stalls reduced 5.2%
>  - compaction stalls reduced 12.3%
>  - page frees reduced 4.9%
> 
> No regressions were observed in requests served per second or request tail
> latency (p99). Both metrics showed directional improvement at higher CPU
> utilization (comparing 85% to 95%).
> 
> Signed-off-by: JP Kobryn (Meta) <jp.kobryn@xxxxxxxxx>

Overall the code looks good but I do wonder if we can add something similar to
folio_add_lru() and if that would be enough.