linux-kernel - Re: [PATCH v5 1/1] mmap_lock: add tracepoints around lock acquisition

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Date:   Mon, 2 Nov 2020 13:49:14 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michel Lespinasse <walken@...gle.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Jann Horn <jannh@...gle.com>,
        Chinwen Chang <chinwen.chang@...iatek.com>,
        Davidlohr Bueso <dbueso@...e.de>,
        David Rientjes <rientjes@...gle.com>,
        Laurent Dufour <ldufour@...ux.ibm.com>
Cc:     Yafang Shao <laoar.shao@...il.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v5 1/1] mmap_lock: add tracepoints around lock acquisition

On 10/28/20 12:07 AM, Axel Rasmussen wrote:
> The goal of these tracepoints is to be able to debug lock contention
> issues. This lock is acquired on most (all?) mmap / munmap / page fault
> operations, so a multi-threaded process which does a lot of these can
> experience significant contention.
> 
> We trace just before we start acquisition, when the acquisition returns
> (whether it succeeded or not), and when the lock is released (or
> downgraded). The events are broken out by lock type (read / write).
> 
> The events are also broken out by memcg path. For container-based
> workloads, users often think of several processes in a memcg as a single
> logical "task", so collecting statistics at this level is useful.
> 
> The end goal is to get latency information. This isn't directly included
> in the trace events. Instead, users are expected to compute the time
> between "start locking" and "acquire returned", using e.g. synthetic
> events or BPF. The benefit we get from this is simpler code.
> 
> Because we use tracepoint_enabled() to decide whether or not to trace,
> this patch has effectively no overhead unless tracepoints are enabled at
> runtime. If tracepoints are enabled, there is a performance impact, but
> how much depends on exactly what e.g. the BPF program does.
> 
> [ rostedt@...dmis.org: in-depth examples of tracepoint_enabled() usage,
> 	and per-cpu-per-context buffer design ]

Great, thanks Steven.

> Signed-off-by: Axel Rasmussen <axelrasmussen@...gle.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>