linux-kernel - [PATCH 3/5] rseq: Lower default slice extension

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Message-ID: <20260121143208.073200729@infradead.org>
Date: Wed, 21 Jan 2026 15:25:00 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: tglx@...utronix.de,
 mathieu.desnoyers@...icios.com
Cc: linux-kernel@...r.kernel.org,
 peterz@...radead.org,
 paulmck@...nel.org,
 boqun.feng@...il.com,
 corbet@....net,
 prakash.sangappa@...cle.com,
 vineethr@...ux.ibm.com,
 kprateek.nayak@....com,
 rostedt@...dmis.org,
 bigeasy@...utronix.de,
 arnd@...db.de,
 rdunlap@...radead.org,
 rongevarg@...il.com,
 longman@...hat.com
Subject: [PATCH 3/5] rseq: Lower default slice extension

Change the minimum slice extension to 5 usec.

Since slice_test selftest reaches a staggering ~350 nsec extension:

Task: slice_test    Mean: 350.266 ns
  Latency (us)    | Count
  ------------------------------
  EXPIRED         | 238
  0 us            | 143189
  1 us            | 167
  2 us            | 26
  3 us            | 11
  4 us            | 28
  5 us            | 31
  6 us            | 22
  7 us            | 23
  8 us            | 32
  9 us            | 16
  10 us           | 35

Lower the minimal (and default) value to 5 usecs -- which is still massive.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 Documentation/userspace-api/rseq.rst |    2 +-
 kernel/rseq.c                        |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

--- a/Documentation/userspace-api/rseq.rst
+++ b/Documentation/userspace-api/rseq.rst
@@ -79,7 +79,7 @@ slice extension by setting rseq::slice_c
 interrupted and the interrupt results in a reschedule request in the
 kernel, then the kernel can grant a time slice extension and return to
 userspace instead of scheduling out. The length of the extension is
-determined by debugfs:rseq/slice_ext_nsec. The default value is 10 usec; which
+determined by debugfs:rseq/slice_ext_nsec. The default value is 5 usec; which
 is the minimum value. It can be incremented to 50 usecs, however doing so
 can/will affect the minimum scheduling latency.
 
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -519,7 +519,7 @@ struct slice_timer {
 	void		*cookie;
 };
 
-static const unsigned int rseq_slice_ext_nsecs_min = 10 * NSEC_PER_USEC;
+static const unsigned int rseq_slice_ext_nsecs_min =  5 * NSEC_PER_USEC;
 static const unsigned int rseq_slice_ext_nsecs_max = 50 * NSEC_PER_USEC;
 unsigned int rseq_slice_ext_nsecs __read_mostly = rseq_slice_ext_nsecs_min;
 static DEFINE_PER_CPU(struct slice_timer, slice_timer);