linux-kernel - Re: [PATCH v2] mm: mmap_lock: optimize mmap

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Message-ID: <Z0Yhivws5XSeme68@google.com>
Date: Tue, 26 Nov 2024 19:29:14 +0000
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...nel.org>,
	Muchun Song <muchun.song@...ux.dev>,
	Vlastimil Babka <vbabka@...e.cz>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
	Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH v2] mm: mmap_lock: optimize mmap_lock tracepoints

On Mon, Nov 25, 2024 at 09:16:17AM -0800, 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 resolve 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>

Acked-by: Roman Gushchin <roman.gushchin@...ux.dev>

Thanks!