linux-kernel - Re: [PATCH] mm: mmap_lock: optimize mmap

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Message-ID: <a2aa5dbl2lwi3q3qzyds3lllgbcgruxjzwzwhnolg63i4swcus@lwtetk5wra44>
Date: Sat, 23 Nov 2024 13:15:41 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Tejun Heo <tj@...nel.org>, Michal Koutny <mkoutny@...e.com>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Muchun Song <muchun.song@...ux.dev>, 
	Axel Rasmussen <axelrasmussen@...gle.com>, Steven Rostedt <rostedt@...dmis.org>, 
	Suren Baghdasaryan <surenb@...gle.com>, linux-mm@...ck.org, cgroups@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH] mm: mmap_lock: optimize mmap_lock tracepoints

On Sat, Nov 23, 2024 at 09:38:09AM +0100, Vlastimil Babka wrote:
> On 11/23/24 7:09 AM, Shakeel Butt wrote:
> > We are starting to deploy mmap_lock tracepoint monitoring across our
> > fleet and the early results showed that these tracepoints are consuming
> > significant amount of CPUs in kernfs_path_from_node when enabled.
> > 
> > It seems like the kernel is trying to resolved the cgroup path in the
> > fast path of the locking code path when the tracepoints are enabled. In
> > addition for some application their metrics are regressing when
> > monitoring is enabled.
> > 
> > The cgroup path resolution can be slow and should not be done in the
> > fast path. Most userspace tools, like bpftrace, provides functionality
> > to get the cgroup path from cgroup id, so let's just trace the cgroup
> > id and the users can use better tools to get the path in the slow path.
> > 
> > Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>
> 
> AFAIU this would also remove the lockdep issue that patch [1] is solving
> with RCU conversion. It probably has other benefits on its own too, so
> just FYI. It's definitely better to avoid complex operations to gather
> tracepoint data, if avoidable.
> 
> [1] https://lore.kernel.org/all/20241121175250.EJbI7VMb@linutronix.de/
> 

Thanks for the pointer, I might add a reference to this in the commit
message in next version.