mm/vmpressure: scale the vmpressure window with machine size

From: Tao Cui <[email protected]>

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

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 <[email protected]>

---
 include/linux/vmpressure.h |  2 +-
 mm/vmpressure.c            | 20 +++++++++++++++++---
 2 files changed, 18 insertions(+), 4 deletions(-)

include/linux/vmpressure.h +1 -1

index b4d13457bc2a..09111f5bdc88 100644
@@ -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);
 

mm/vmpressure.c +17 -4

index 9629240d77ad..4c8671273c79 100644
@@ -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

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/[email protected]/ [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.

Quoted 10 lines

From: Tao Cui <[email protected]>

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.

Quoted 7 lines

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...

Quoted 77 lines

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 <[email protected]>

---
 include/linux/vmpressure.h |  2 +-
 mm/vmpressure.c            | 20 +++++++++++++++++---
 2 files changed, 18 insertions(+), 4 deletions(-)

include/linux/vmpressure.h +1 -1

index b4d13457bc2a..09111f5bdc88 100644
@@ -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);
 

mm/vmpressure.c +17 -4

index 9629240d77ad..4c8671273c79 100644
@@ -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
--

Quoted 2 lines

2.43.0

Cheers, Lorenzo