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sched/balancing: Switch the 'DEFINE_SPINLOCK(balancing)' spinlock into an 'atomic_t sched_balance_running' flag

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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: Shrikanth Hegde <sshegde@linux.ibm.com>
Link: https://lore.kernel.org/r/20240308105901.1096078-2-mingo@kernel.org
This commit is contained in:
Ingo Molnar 2024-03-08 11:58:52 +01:00
parent 855684c7d9
commit 214c1b7f13
1 changed files with 16 additions and 3 deletions
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19
kernel/sched/fair.c
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@ -11633,7 +11633,20 @@ out_unlock:
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;