linux-kernel - Re: [PATCH v6 0/5] reduce tasklist_lock hold time on exit and do some pid cleanup

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Message-ID: <572h7chpl6z5ldizh3oh63olcyb6vyh4k4lq7ozmpwnqybkfxo@qarxk4rdndig>
Date: Thu, 6 Feb 2025 12:14:34 -0500
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: ebiederm@...ssion.com, oleg@...hat.com, brauner@...nel.org,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 0/5] reduce tasklist_lock hold time on exit and do
 some pid cleanup

* Mateusz Guzik <mjguzik@...il.com> [250206 11:44]:
> I fixed fat-fingering and touchedup some commit messages. git diff
> between the old and new branch does not show any code changes.


Thanks for doing these changes.

Acked-by: Liam R. Howlett <Liam.Howlett@...cle.com>

> 
> hopefully we will be done here after this iteration 8->
> 
> old cover letter:
> 
> The clone side contends against exit side in a way which avoidably
> exacerbates the problem by the latter waiting on locks held by the
> former while holding the tasklist_lock.
> 
> Whacking this for both add_device_randomness and pids allocation gives
> me a 15% speed up for thread creation/destruction in a 24-core vm.
> 
> The random patch is worth about 4%.
> 
> The new bottleneck is pidmap_lock itself, with the biggest problem being
> the allocation itself taking the lock *twice*.
> 
> Bench (plop into will-it-scale):
> $ cat tests/threadspawn1.c
> 
> char *testcase_description = "Thread creation and teardown";
> 
> static void *worker(void *arg)
> {
> 	return (NULL);
> }
> 
> void testcase(unsigned long long *iterations, unsigned long nr)
> {
> 	pthread_t thread;
> 	int error;
> 
> 	while (1) {
> 		error = pthread_create(&thread, NULL, worker, NULL);
> 		assert(error == 0);
> 		error = pthread_join(thread, NULL);
> 		assert(error == 0);
> 		(*iterations)++;
> 	}
> }
> 
> v6:
> - expand on the commit message in 3/5 pid: sprinkle tasklist_lock asserts
> - move fat-fingered free_pids call to the right patch
> 
> v5:
> - whack scripts/selinux/genheaders/genheaders which accidentally got in
> - rebased on next-20250205
> 
> v4:
> - justify moving get_pid in the commit message with a one-liner
> - drop the tty unref patch -- it is completely optional and Oleg has his
>   own variant
> - add the ACK by Oleg
> 
> v3:
> - keep procfs flush where it was, instead hoist get_pid outside of the
>   lock
> - make detach_pid et al accept an array argument of pids to populate
> - sprinkle asserts
> - drop irq trips around pidmap_lock
> - move tty unref outside of tasklist_lock
> 
> 
> 
> Mateusz Guzik (5):
>   exit: perform add_device_randomness() without tasklist_lock
>   exit: hoist get_pid() in release_task() outside of tasklist_lock
>   pid: sprinkle tasklist_lock asserts
>   pid: perform free_pid() calls outside of tasklist_lock
>   pid: drop irq disablement around pidmap_lock
> 
>  include/linux/pid.h |  7 ++--
>  kernel/exit.c       | 36 +++++++++++++-------
>  kernel/pid.c        | 82 +++++++++++++++++++++++++--------------------
>  kernel/sys.c        | 14 +++++---
>  4 files changed, 82 insertions(+), 57 deletions(-)
> 
> -- 
> 2.43.0
>