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Date:   Tue, 13 Oct 2020 12:42:15 -0700 (PDT)
From:   David Rientjes <rientjes@google.com>
To:     Axel Rasmussen <axelrasmussen@google.com>
cc:     Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michel Lespinasse <walken@google.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Daniel Jordan <daniel.m.jordan@oracle.com>,
        Laurent Dufour <ldufour@linux.ibm.com>,
        Jann Horn <jannh@google.com>,
        Chinwen Chang <chinwen.chang@mediatek.com>,
        Yafang Shao <laoar.shao@gmail.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v3 2/2] mmap_lock: add tracepoints around lock
 acquisition
In-Reply-To: <20201009220524.485102-3-axelrasmussen@google.com>
Message-ID: <alpine.DEB.2.23.453.2010131242010.2883230@chino.kir.corp.google.com>
References: <20201009220524.485102-1-axelrasmussen@google.com> <20201009220524.485102-3-axelrasmussen@google.com>
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On Fri, 9 Oct 2020, 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.
> 
> Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>

Acked-by: David Rientjes <rientjes@google.com>