Re: [PATCH] mm/vmpressure: scale the vmpressure window with machine size

[Tao Cui]

在 2026/7/24 15:30, Lorenzo Stoakes (ARM) 写道:
> No thanks.
> 
> Firstly, a change like this should be RFC, especially from somebody who has not
> established trust in the community yet.
> 
> Secondly, this is more or less identical to a previously rejected patch ([0]) so
> seems to be plagiarism since you don't reference that at all.
> 
> Thirdly, this is the 2nd time I've seen a submission copying that patch
> unacknowledged (see my response to the last one at [1]).
> 
> I suspect actually that you're not plagiarising here but rather sending
> unacknowledged LLM-generated code in violation of kernel guidelines ([2]) -
> since the TODO must be a pretty obvious trigger for agents generating this
> patch.
> 
> But regardless of which it is, this patch is not welcome.
> 
> Thanks, Lorenzo
> 
> [0]:https://lore.kernel.org/all/20260227221555.29969-1-mcq@xxxxxxxxxxx/
> [1]:https://lore.kernel.org/linux-mm/aljYxLusTzZkTfnH@lucifer/
> [2]:https://docs.kernel.org/process/coding-assistants.html
> 
> Sorry but no - this is a subtle problem that requires expertise and a strong
> argument in favour with data to back it not... this.
> 
Understood. Thanks for the review, and for the pointer to the earlier
patch [0]. I should have searched for prior attempts and referenced it
rather than sending this; my apologies. Withdrawing it.

I've read the coding-assistants guidelines [2] and will follow them going
forward.

Thanks,
Tao Cui
> On Fri, Jul 24, 2026 at 01:43:05PM +0800, Tao Cui wrote:
>> From: Tao Cui <cuitao@xxxxxxxxxx>
>>
>> vmpressure_win -- the number of pages the reclaimer must scan before
>> socket pressure is re-evaluated -- has been a fixed 512 pages (2 MB) since
>> vmpressure was introduced.  The reclaimer scans more pages on a larger
>> machine, so the window is reached far more often there and socket pressure
>> is re-armed every handful of scanned pages.  The comment at the definition
>> has asked for the window to scale with machine size "as we do for vmstat
>> thresholds" for over a decade.
>>
>> Scale it the same way calculate_normal_threshold() does: logarithmically
>> with memory (fls of memory in 128 MB units), computed once in a
>> subsys_initcall once totalram_pages() is known.  A machine under 128 MB
>> keeps the historical 512 pages; the window then grows by SWAP_CLUSTER_MAX
>> * 16 per doubling of memory.
>>
>> Why this matters: the scanned/reclaimed ratio that drives socket pressure
> 
> "Why this matters"... oh I wonder where I've heard that kind of phrasing
> before...
> 
>> is averaged over the window, and the window rate-limits the evaluation.
>> With a fixed 2 MB window the evaluation runs the same number of times
>> regardless of machine size, which is disproportionately many on a large
>> machine.  Measured by cold-booting one VM at each size and running the
>> same cgroup-bound reclaim workload (so the page count is identical across
>> sizes):
>>
>>     config   scaled_win   pages scanned   512-win evals   scaled evals
>>     4 GB     3072         11.9 M          23267           3877
>>     8 GB     3584         11.9 M          23306           3329
>>     16 GB    4096         11.9 M          23281           2910
>>     32 GB    4608         11.9 M          23268           2585
>>     64 GB    5120         11.9 M          23281           2328
>>
>> For the same reclaim work the fixed window evaluates ~23k times at every
>> machine size; the scaled window evaluates fewer times the larger the
>> machine -- a 6x reduction at 4 GB growing to 10x at 64 GB (and the
>> logarithmic growth continues: ~11x projected at 128 GB).  Each evaluation
>> takes the per-memcg sr_lock and may write the socket_pressure seqlock, so
>> on larger machines with more memcgs under pressure this is real overhead
>> the fixed window pays needlessly.
>>
>> The default stays 512 until the initcall runs, so early-boot reclaim is
>> unchanged.
>>
>> Signed-off-by: Tao Cui <cuitao@xxxxxxxxxx>
>> ---
>>  include/linux/vmpressure.h |  2 +-
>>  mm/vmpressure.c            | 20 +++++++++++++++++---
>>  2 files changed, 18 insertions(+), 4 deletions(-)
>>
>> diff --git a/include/linux/vmpressure.h b/include/linux/vmpressure.h
>> index b4d13457bc2a..09111f5bdc88 100644
>> --- a/include/linux/vmpressure.h
>> +++ b/include/linux/vmpressure.h
>> @@ -51,7 +51,7 @@ extern struct vmpressure *memcg_to_vmpressure(struct mem_cgroup *memcg);
>>  extern struct mem_cgroup *vmpressure_to_memcg(struct vmpressure *vmpr);
>>
>>  /* Shared with the v1 vmpressure block in mm/memcontrol-v1.c. */
>> -extern const unsigned long vmpressure_win;
>> +extern unsigned long vmpressure_win;
>>  extern enum vmpressure_levels vmpressure_calc_level(unsigned long scanned,
>>  						    unsigned long reclaimed);
>>
>> diff --git a/mm/vmpressure.c b/mm/vmpressure.c
>> index 9629240d77ad..4c8671273c79 100644
>> --- a/mm/vmpressure.c
>> +++ b/mm/vmpressure.c
>> @@ -31,10 +31,24 @@
>>   * As the vmscan reclaimer logic works with chunks which are multiple of
>>   * SWAP_CLUSTER_MAX, it makes sense to use it for the window size as well.
>>   *
>> - * TODO: Make the window size depend on machine size, as we do for vmstat
>> - * thresholds. Currently we set it to 512 pages (2MB for 4KB pages).
>> + * Scale the window with machine size, the way vmstat thresholds do: on a
>> + * larger machine the reclaimer scans more pages, so a fixed window would
>> + * re-evaluate pressure every handful of pages. The scaling is logarithmic
>> + * (fls, like calculate_normal_threshold()), keeping the growth moderate.
>> + * The default is the historical 512 pages; an early initcall applies the
>> + * scaling once totalram_pages() is known.
>>   */
>> -const unsigned long vmpressure_win = SWAP_CLUSTER_MAX * 16;
>> +unsigned long vmpressure_win __read_mostly = SWAP_CLUSTER_MAX * 16;
>> +
>> +static int __init vmpressure_init_window(void)
>> +{
>> +	unsigned long mem128m;	/* machine memory in 128MB units */
>> +
>> +	mem128m = totalram_pages() >> (27 - PAGE_SHIFT);
>> +	vmpressure_win = SWAP_CLUSTER_MAX * 16 * (1 + fls(mem128m));
>> +	return 0;
>> +}
>> +subsys_initcall(vmpressure_init_window);
>>
>>  /*
>>   * These thresholds are used when we account memory pressure through
>> --
>> 2.43.0
>>
> 
> Cheers, Lorenzo