6.12-stable patches

added patches:
	softirq-allow-raising-sched_softirq-from-smp-call-function-on-rt-kernel.patch

diff --git a/queue-6.12/series b/queue-6.12/series
index 5199b7d666445dbda4015857fe2599aaab3b825a..c6438680a9acdb495b9d88eb41fedb284646753a 100644 (file)
--- a/queue-6.12/series
+++ b/queue-6.12/series
@@ -434,7 +434,6 @@ drm-xe-devcoredump-add-ascii85-dump-helper-function.patch
 drm-xe-guc-copy-guc-log-prior-to-dumping.patch
 drm-xe-forcewake-add-a-helper-xe_force_wake_ref_has_.patch
 drm-xe-devcoredump-update-handling-of-xe_force_wake_.patch
-drm-amd-display-update-interface-to-check-uclk-dpm.patch
 drm-amd-display-add-option-to-retrieve-detile-buffer.patch
 sched-fix-warning-in-sched_setaffinity.patch
 sched-core-remove-the-unnecessary-need_resched-check.patch
@@ -465,3 +464,4 @@ s390-pci-fix-leak-of-struct-zpci_dev-when-zpci_add_device-fails.patch
 alsa-hda-realtek-fix-spelling-mistake-firelfy-firefly.patch
 timekeeping-remove-config_debug_timekeeping.patch
 clocksource-make-negative-motion-detection-more-robust.patch
+softirq-allow-raising-sched_softirq-from-smp-call-function-on-rt-kernel.patch

diff --git a/queue-6.12/softirq-allow-raising-sched_softirq-from-smp-call-function-on-rt-kernel.patch b/queue-6.12/softirq-allow-raising-sched_softirq-from-smp-call-function-on-rt-kernel.patch
new file mode 100644 (file)
index 0000000..1a86ff9
--- /dev/null
+++ b/queue-6.12/softirq-allow-raising-sched_softirq-from-smp-call-function-on-rt-kernel.patch
@@ -0,0 +1,85 @@
+From 6675ce20046d149e1e1ffe7e9577947dee17aad5 Mon Sep 17 00:00:00 2001
+From: K Prateek Nayak <kprateek.nayak@amd.com>
+Date: Tue, 19 Nov 2024 05:44:29 +0000
+Subject: softirq: Allow raising SCHED_SOFTIRQ from SMP-call-function on RT kernel
+
+From: K Prateek Nayak <kprateek.nayak@amd.com>
+
+commit 6675ce20046d149e1e1ffe7e9577947dee17aad5 upstream.
+
+do_softirq_post_smp_call_flush() on PREEMPT_RT kernels carries a
+WARN_ON_ONCE() for any SOFTIRQ being raised from an SMP-call-function.
+Since do_softirq_post_smp_call_flush() is called with preempt disabled,
+raising a SOFTIRQ during flush_smp_call_function_queue() can lead to
+longer preempt disabled sections.
+
+Since commit b2a02fc43a1f ("smp: Optimize
+send_call_function_single_ipi()") IPIs to an idle CPU in
+TIF_POLLING_NRFLAG mode can be optimized out by instead setting
+TIF_NEED_RESCHED bit in idle task's thread_info and relying on the
+flush_smp_call_function_queue() in the idle-exit path to run the
+SMP-call-function.
+
+To trigger an idle load balancing, the scheduler queues
+nohz_csd_function() responsible for triggering an idle load balancing on
+a target nohz idle CPU and sends an IPI. Only now, this IPI is optimized
+out and the SMP-call-function is executed from
+flush_smp_call_function_queue() in do_idle() which can raise a
+SCHED_SOFTIRQ to trigger the balancing.
+
+So far, this went undetected since, the need_resched() check in
+nohz_csd_function() would make it bail out of idle load balancing early
+as the idle thread does not clear TIF_POLLING_NRFLAG before calling
+flush_smp_call_function_queue(). The need_resched() check was added with
+the intent to catch a new task wakeup, however, it has recently
+discovered to be unnecessary and will be removed in the subsequent
+commit after which nohz_csd_function() can raise a SCHED_SOFTIRQ from
+flush_smp_call_function_queue() to trigger an idle load balance on an
+idle target in TIF_POLLING_NRFLAG mode.
+
+nohz_csd_function() bails out early if "idle_cpu()" check for the
+target CPU, and does not lock the target CPU's rq until the very end,
+once it has found tasks to run on the CPU and will not inhibit the
+wakeup of, or running of a newly woken up higher priority task. Account
+for this and prevent a WARN_ON_ONCE() when SCHED_SOFTIRQ is raised from
+flush_smp_call_function_queue().
+
+Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
+Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
+Link: https://lore.kernel.org/r/20241119054432.6405-2-kprateek.nayak@amd.com
+Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
+---
+ kernel/softirq.c |   15 +++++++++++----
+ 1 file changed, 11 insertions(+), 4 deletions(-)
+
+--- a/kernel/softirq.c
++++ b/kernel/softirq.c
+@@ -280,17 +280,24 @@ static inline void invoke_softirq(void)
+               wakeup_softirqd();
+ }
+ 
++#define SCHED_SOFTIRQ_MASK    BIT(SCHED_SOFTIRQ)
++
+ /*
+  * flush_smp_call_function_queue() can raise a soft interrupt in a function
+- * call. On RT kernels this is undesired and the only known functionality
+- * in the block layer which does this is disabled on RT. If soft interrupts
+- * get raised which haven't been raised before the flush, warn so it can be
++ * call. On RT kernels this is undesired and the only known functionalities
++ * are in the block layer which is disabled on RT, and in the scheduler for
++ * idle load balancing. If soft interrupts get raised which haven't been
++ * raised before the flush, warn if it is not a SCHED_SOFTIRQ so it can be
+  * investigated.
+  */
+ void do_softirq_post_smp_call_flush(unsigned int was_pending)
+ {
+-      if (WARN_ON_ONCE(was_pending != local_softirq_pending()))
++      unsigned int is_pending = local_softirq_pending();
++
++      if (unlikely(was_pending != is_pending)) {
++              WARN_ON_ONCE(was_pending != (is_pending & ~SCHED_SOFTIRQ_MASK));
+               invoke_softirq();
++      }
+ }
+ 
+ #else /* CONFIG_PREEMPT_RT */