Re: [PATCH] mm/kmemleak: rely on rcu for task stack scanning

[Qian Cai <cai@lca.pw>]

On Thu, Aug 20, 2020 at 06:27:50PM -0700, Davidlohr Bueso wrote:
> On Thu, 20 Aug 2020, Qian Cai wrote:
> 
> > On Thu, Aug 20, 2020 at 01:39:02PM -0700, Davidlohr Bueso wrote:
> > > kmemleak_scan() currently relies on the big tasklist_lock
> > > hammer to stabilize iterating through the tasklist. Instead,
> > > this patch proposes simply using rcu along with the rcu-safe
> > > for_each_process_thread flavor (without changing scan semantics),
> > > which doesn't make use of next_thread/p->thread_group and thus
> > > cannot race with exit. Furthermore, any races with fork()
> > > and not seeing the new child should be benign as it's not
> > > running yet and can also be detected by the next scan.
> > 
> > It is not entirely clear to me what problem the patch is trying to solve. If
> > this is about performance, we will probably need some number.
> 
> So in this case avoiding the tasklist_lock could prove beneficial for performance
> considering the scan operation is done periodically. I have seen improvements
> of 30%-ish when doing similar replacements on very pathological microbenchmarks
> (ie stressing get/setpriority(2)).
> 
> However my main motivation is that it's one less user of the global lock,
> something that Linus has long time wanted to see gone eventually (if ever)
> even if the traditional fairness issues has been dealt with now with qrwlocks.
> Of course this is a very long ways ahead. This patch also kills another user
> of the deprecated tsk->thread_group.

This makes thing clearer.

Reviewed-by: Qian Cai <cai@lca.pw>