Re: [PATCH] mm: mmap_lock: optimize mmap_lock tracepoints

[Shakeel Butt]

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@xxxxxxxxx>
> 
> 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@xxxxxxxxxxxxx/
> 

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