[RFC,04/14] x86/thread_info: Introduce TIF_NOTIFY_IPI flag

Commit Message

K Prateek Nayak Feb. 20, 2024, 5:14 p.m. UTC

From: "Gautham R. Shenoy" <[e-mail]>

Add support for TIF_NOTIFY_IPI on x86. With TIF_NOTIFY_IPI, a sender
sending an IPI to an idle CPU in TIF_POLLING mode will set the
TIF_NOTIFY_IPI flag in the target's idle tasks's thread_info to pull the
CPU out of idle, as opposed to setting TIF_NEED_RESCHED previously. This
avoids spurious calls to schedule_idle() in cases where an IPI does not
necessarily wake up a task on the idle CPU.

IPI throughput measured using a modified version of Anton Blanchard's
ipistorm benchmark [1], configured to measure time taken to perform a
fixed number of smp_call_function_single() (with wait set to 1),
improves significantly with TIF_NOTIFY_IPI on a dual socket Ampere Server
(2 x 64C) with the benchmark time reducing to less than half for
100000 IPIs between two CPUs. (Note: Only WFI idle mode was left enabled
during testing to reduce variance)

cmdline: insmod ipistorm.ko numipi=100000 single=1 offset=8 cpulist=8 wait=1

  ==================================================================
  Test          : ipistorm (modified)
  Units         : Normalized runtime
  Interpretation: Lower is better
  Statistic     : AMean
  ==================================================================
  kernel:                       	time [pct imp]
  tip:sched/core                	1.00 [0.00]
  tip:sched/core + TIF_NOTIFY_IPI	0.41 [59.29]

tip:sched/core was at tag "sched-core-2024-01-08" at the time of
testing.

[ prateek: Split the changes into a separate patch, commit log ]

Link: https://github.com/antonblanchard/ipistorm [1]
Cc: Thomas Gleixner <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: "H. Peter Anvin" <[e-mail]>
Cc: "Rafael J. Wysocki" <[e-mail]>
Cc: Daniel Lezcano <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Juri Lelli <[e-mail]>
Cc: Vincent Guittot <[e-mail]>
Cc: Dietmar Eggemann <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Cc: Ben Segall <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Cc: Daniel Bristot de Oliveira <[e-mail]>
Cc: Valentin Schneider <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: Tim Chen <[e-mail]>
Cc: David Vernet <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Gautham R. Shenoy <[e-mail]>
Co-developed-by: K Prateek Nayak <[e-mail]>
Signed-off-by: K Prateek Nayak <[e-mail]>
---
 arch/x86/include/asm/thread_info.h | 2 ++
 1 file changed, 2 insertions(+)

Patch

diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
index d63b02940747..90cdce971397 100644
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -83,6 +83,7 @@  struct thread_info {
 #define TIF_NEED_RESCHED	3	/* rescheduling necessary */
 #define TIF_SINGLESTEP		4	/* reenable singlestep on user return*/
 #define TIF_SSBD		5	/* Speculative store bypass disable */
+#define TIF_NOTIFY_IPI		6	/* Pending IPI on TIF_POLLLING idle CPU */
 #define TIF_SPEC_IB		9	/* Indirect branch speculation mitigation */
 #define TIF_SPEC_L1D_FLUSH	10	/* Flush L1D on mm switches (processes) */
 #define TIF_USER_RETURN_NOTIFY	11	/* notify kernel of userspace return */
@@ -106,6 +107,7 @@  struct thread_info {
 #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
 #define _TIF_SINGLESTEP		(1 << TIF_SINGLESTEP)
 #define _TIF_SSBD		(1 << TIF_SSBD)
+#define _TIF_NOTIFY_IPI		(1 << TIF_NOTIFY_IPI)
 #define _TIF_SPEC_IB		(1 << TIF_SPEC_IB)
 #define _TIF_SPEC_L1D_FLUSH	(1 << TIF_SPEC_L1D_FLUSH)
 #define _TIF_USER_RETURN_NOTIFY	(1 << TIF_USER_RETURN_NOTIFY)