Re: [PATCH v4] mm/vmpressure: skip socket pressure for costly order reclaim

[Vlastimil Babka (SUSE)]

On 4/6/26 21:50, JP Kobryn (Meta) wrote:
> When reclaim is triggered by high order allocations on a fragmented system,
> vmpressure() can report poor reclaim efficiency even though the system has
> plenty of free memory. This is because many pages are scanned, but few are
> found to actually reclaim - the pages are actively in use and don't need to
> be freed. The resulting scan:reclaim ratio causes vmpressure() to assert
> socket pressure, throttling TCP throughput unnecessarily.
> 
> Costly order allocations (above PAGE_ALLOC_COSTLY_ORDER) rely heavily on
> compaction to succeed, so poor reclaim efficiency at these orders does not
> necessarily indicate memory pressure. The kernel already treats this order
> as the boundary where reclaim is no longer expected to succeed and
> compaction may take over.
> 
> Make vmpressure() order-aware through an additional parameter sourced from
> scan_control at existing call sites. Socket pressure is now only asserted
> when order <= PAGE_ALLOC_COSTLY_ORDER.
> 
> Memcg reclaim is unaffected since try_to_free_mem_cgroup_pages() always
> uses order 0, which passes the filter unconditionally. Similarly,
> vmpressure_prio() now passes order 0 internally when calling vmpressure(),
> ensuring critical pressure from low reclaim priority is not suppressed by
> the order filter.
> 
> The patch was motivated by a case of impacted net throughput in production.
> On one affected host, the memory state at the time showed ~15GB available,
> zero cgroup pressure, and the following buddyinfo state:
> 
> Order FreePages
> 0:    133,970
> 1:    29,230
> 2:    17,351
> 3:    18,984
> 7+:   0
> 
> Using bpf, it was found that 94% of vmpressure calls on this host were from
> order-7 kswapd reclaim.
> 
> TCP minimum recv window is rcv_ssthresh:19712.
> 
> Before patch:
> 723 out of 3,843 (19%) TCP connections stuck at minimum recv window
> 
> After live-patching and ~30min elapsed:
> 0 out of 3,470 TCP connections stuck at minimum recv window
> 
> Signed-off-by: JP Kobryn (Meta) <jp.kobryn@xxxxxxxxx>
> Reviewed-by: Rik van Riel <riel@xxxxxxxxxxx>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> Acked-by: Jakub Kicinski <kuba@xxxxxxxxxx>

Acked-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>