vgsp-gr r552: retire-wait settle window 300 -> 100 ms (launch p50 1119 -> 651 ms)

Measured A/B on real boots (same module, same client set, only
gsp_phys_gr_settle_ms varied):
 - r549 (default 300): 32 launches, min 862 / p50 1119 / p90 2363 ms;
   retire wait 680-901 ms per launch ("(settled)", 1..64 of 1000+
   tracked pages changed).
 - r551 (100): 22 launches, min 476 / p50 651.5 / p90 1010 / max 1428 ms;
   retire wait 231-247 ms. Battery 12/15, 0 hangs (unchanged profile;
   the 3 fails are the streams2/streams_events/memset2d readback class).

Why the wait was the dominant cost: a read-mostly kernel writes far fewer
pages than the launch's tracked set, so the "changed >= n_all" shortcut
never fires and every launch settles through the content-stable rule,
which paid the full window plus scan overhead. The stability verdict is
still hash-gated (m180); only the minimum stable duration shrinks, and
the battery correctness profile is unchanged. 300 remains available
through the module parameter.

1 files changed, 13 insertions, 3 deletions

diff --git a/kernel/gsp_phys_booter.c b/kernel/gsp_phys_booter.c
index 734ebf2..82c88dc 100644
--- a/kernel/gsp_phys_booter.c
+++ b/kernel/gsp_phys_booter.c
@@ -152,9 +152,19 @@ static inline bool gsp_phys_gr_copywit_enabled(void)
  *    "ENGINE STILL WRITING" proves the copy-out shipped before the stores
  *    landed (the fix is a longer settle window); "stable" proves the engine
  *    genuinely stopped, which needs a different treatment.
- *  - gsp_phys_gr_settle_ms (default 300): the content-stable window the
+ *  - gsp_phys_gr_settle_ms (default 100): the content-stable window the
  *    partial-changed rule requires before it declares the stores settled.
  *    Raise it to give a still-writing engine its time (A/B in one cycle).
+ *
+ *    r552 (2026-10-23): measured A/B on real boots. Every launch with a
+ *    partial changed-page count (read-mostly kernels: 1..64 of 1000+ pages
+ *    written, so the changed>=n_all shortcut can never fire) paid the full
+ *    window plus scan overhead: 683-901 ms per launch at 300 ms. At 100 ms
+ *    the same boots measured 231-247 ms of retire wait and the launch
+ *    latency p50 dropped 1119 ms -> 651 ms (n=22 launches; baseline 684 ms)
+ *    with the battery unchanged (12/15, 0 hangs) and a green 2-round soak
+ *    on the next boot. The content-stability hash still gates the verdict;
+ *    only the minimum stable duration shrank.
  */
 static bool gsp_phys_gr_late_probe_param;
 module_param_named(gsp_phys_gr_late_probe, gsp_phys_gr_late_probe_param,
@@ -162,11 +172,11 @@ module_param_named(gsp_phys_gr_late_probe, gsp_phys_gr_late_probe_param,
 MODULE_PARM_DESC(gsp_phys_gr_late_probe,
 		 "Diagnostic: after the GR retire wait, re-scan the tracked pages +500 ms later and log whether the engine is still writing (default 0)");
 
-static int gsp_phys_gr_settle_ms_param = 300;
+static int gsp_phys_gr_settle_ms_param = 100;
 module_param_named(gsp_phys_gr_settle_ms, gsp_phys_gr_settle_ms_param,
 		   int, 0644);
 MODULE_PARM_DESC(gsp_phys_gr_settle_ms,
-		 "GR retire wait: content-stable window (ms) required before a partial changed-page count counts as settled (default 300)");
+		 "GR retire wait: content-stable window (ms) required before a partial changed-page count counts as settled (default 100; 300 was the pre-r552 value)");
 
 static inline u32 gsp_phys_gr_settle_iters(void)
 {