From: Ingo Molnar <mingo@kernel.org>
To: linux-kernel@vger.kernel.org
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Peter Zijlstra <peterz@infradead.org>,
Shrikanth Hegde <sshegde@linux.ibm.com>,
Valentin Schneider <vschneid@redhat.com>,
Vincent Guittot <vincent.guittot@linaro.org>
Subject: [PATCH 01/10] sched/balancing: Switch the 'DEFINE_SPINLOCK(balancing)' spinlock into an 'atomic_t sched_balance_running' flag
Date: Fri, 8 Mar 2024 11:58:52 +0100 [thread overview]
Message-ID: <20240308105901.1096078-2-mingo@kernel.org> (raw)
In-Reply-To: <20240308105901.1096078-1-mingo@kernel.org>
The 'balancing' spinlock added in:
08c183f31bdb ("[PATCH] sched: add option to serialize load balancing")
... is taken when the SD_SERIALIZE flag is set in a domain, but in reality it
is a glorified global atomic flag serializing the load-balancing of
those domains.
It doesn't have any explicit locking semantics per se: we just
spin_trylock() it.
Turn it into a ... global atomic flag. This makes it more
clear what is going on here, and reduces overhead and code
size a bit:
# kernel/sched/fair.o: [x86-64 defconfig]
text data bss dec hex filename
60730 2721 104 63555 f843 fair.o.before
60718 2721 104 63543 f837 fair.o.after
Also document the flag a bit.
No change in functionality intended.
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Shrikanth Hegde <sshegde@linux.ibm.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
---
kernel/sched/fair.c | 19 ++++++++++++++++---
1 file changed, 16 insertions(+), 3 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6a16129f9a5c..2ef89b36aed1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11633,7 +11633,20 @@ static int active_load_balance_cpu_stop(void *data)
return 0;
}
-static DEFINE_SPINLOCK(balancing);
+/*
+ * This flag serializes load-balancing passes over large domains
+ * (above the NODE topology level) - only one load-balancing instance
+ * may run at a time, to reduce overhead on very large systems with
+ * lots of CPUs and large NUMA distances.
+ *
+ * - Note that load-balancing passes triggered while another one
+ * is executing are skipped and not re-tried.
+ *
+ * - Also note that this does not serialize rebalance_domains()
+ * execution, as non-SD_SERIALIZE domains will still be
+ * load-balanced in parallel.
+ */
+static atomic_t sched_balance_running = ATOMIC_INIT(0);
/*
* Scale the max load_balance interval with the number of CPUs in the system.
@@ -11711,7 +11724,7 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
need_serialize = sd->flags & SD_SERIALIZE;
if (need_serialize) {
- if (!spin_trylock(&balancing))
+ if (atomic_cmpxchg_acquire(&sched_balance_running, 0, 1))
goto out;
}
@@ -11729,7 +11742,7 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
interval = get_sd_balance_interval(sd, busy);
}
if (need_serialize)
- spin_unlock(&balancing);
+ atomic_set_release(&sched_balance_running, 0);
out:
if (time_after(next_balance, sd->last_balance + interval)) {
next_balance = sd->last_balance + interval;
--
2.40.1
next prev parent reply other threads:[~2024-03-08 10:59 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-08 10:58 [PATCH -v4 00/10] sched/balancing: Misc updates & cleanups Ingo Molnar
2024-03-08 10:58 ` Ingo Molnar [this message]
2024-03-12 12:00 ` [tip: sched/core] sched/balancing: Switch the 'DEFINE_SPINLOCK(balancing)' spinlock into an 'atomic_t sched_balance_running' flag tip-bot2 for Ingo Molnar
2024-03-15 9:25 ` [PATCH 01/10] " Shrikanth Hegde
2024-03-08 10:58 ` [PATCH 02/10] sched/balancing: Remove reliance on 'enum cpu_idle_type' ordering when iterating [CPU_MAX_IDLE_TYPES] arrays in show_schedstat() Ingo Molnar
2024-03-08 13:26 ` Vincent Guittot
2024-03-12 12:00 ` [tip: sched/core] " tip-bot2 for Shrikanth Hegde
2024-03-08 10:58 ` [PATCH 03/10] sched/balancing: Change 'enum cpu_idle_type' to have more natural definitions Ingo Molnar
2024-03-12 12:00 ` [tip: sched/core] " tip-bot2 for Ingo Molnar
2024-03-08 10:58 ` [PATCH 04/10] sched/debug: Increase SCHEDSTAT_VERSION to 16 Ingo Molnar
2024-03-08 16:40 ` Shrikanth Hegde
2024-03-12 10:01 ` Ingo Molnar
2024-03-14 5:21 ` Gautham R. Shenoy
2024-03-14 6:04 ` Swapnil Sapkal
2024-03-15 4:37 ` Shrikanth Hegde
2024-03-15 9:06 ` Swapnil Sapkal
2024-03-15 9:20 ` Shrikanth Hegde
2024-03-15 14:03 ` Swapnil Sapkal
2024-03-12 12:00 ` [tip: sched/core] " tip-bot2 for Ingo Molnar
2024-03-08 10:58 ` [PATCH 05/10] sched/debug: Allow CONFIG_SCHEDSTATS even on !KERNEL_DEBUG kernels Ingo Molnar
2024-03-12 12:00 ` [tip: sched/core] " tip-bot2 for Ingo Molnar
2024-03-08 10:58 ` [PATCH 06/10] sched/balancing: Change comment formatting to not overlap Git conflict marker lines Ingo Molnar
2024-03-12 12:00 ` [tip: sched/core] " tip-bot2 for Ingo Molnar
2024-03-08 10:58 ` [PATCH 07/10] sched/balancing: Fix comments (trying to) refer to NOHZ_BALANCE_KICK Ingo Molnar
2024-03-08 12:03 ` Vincent Guittot
2024-03-12 12:00 ` [tip: sched/core] " tip-bot2 for Ingo Molnar
2024-03-08 10:58 ` [PATCH 08/10] sched/balancing: Update run_rebalance_domains() comments Ingo Molnar
2024-03-08 12:02 ` Vincent Guittot
2024-03-12 12:00 ` [tip: sched/core] " tip-bot2 for Ingo Molnar
2024-03-08 10:59 ` [PATCH 09/10] sched/balancing: Vertically align the comments of 'struct sg_lb_stats' and 'struct sd_lb_stats' Ingo Molnar
2024-03-08 12:01 ` Vincent Guittot
2024-03-12 12:00 ` [tip: sched/core] " tip-bot2 for Ingo Molnar
2024-03-08 10:59 ` [PATCH 10/10] sched/balancing: Update comments in " Ingo Molnar
2024-03-08 12:01 ` Vincent Guittot
2024-03-12 12:00 ` [tip: sched/core] " tip-bot2 for Ingo Molnar
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