From: Peter Zijlstra <peterz@infradead.org>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: Viktor Rosendahl <viktor.rosendahl@gmail.com>,
paulmck@kernel.org, Steven Rostedt <rostedt@goodmis.org>,
Ingo Molnar <mingo@redhat.com>,
linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org
Subject: Re: [PATCH v5 1/4] ftrace: Implement fs notification for tracing_max_latency
Date: Wed, 4 Sep 2019 10:19:19 +0200 [thread overview]
Message-ID: <20190904081919.GA2349@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20190904040039.GB150430@google.com>
On Wed, Sep 04, 2019 at 12:00:39AM -0400, Joel Fernandes wrote:
> [ Resending since I messed up my last email's headers! ]
>
> On Tue, Sep 03, 2019 at 03:25:59PM +0200, Viktor Rosendahl wrote:
> > This patch implements the feature that the tracing_max_latency file,
> > e.g. /sys/kernel/debug/tracing/tracing_max_latency will receive
> > notifications through the fsnotify framework when a new latency is
> > available.
> >
> > One particularly interesting use of this facility is when enabling
> > threshold tracing, through /sys/kernel/debug/tracing/tracing_thresh,
> > together with the preempt/irqsoff tracers. This makes it possible to
> > implement a user space program that can, with equal probability,
> > obtain traces of latencies that occur immediately after each other in
> > spite of the fact that the preempt/irqsoff tracers operate in overwrite
> > mode.
>
> Adding Paul since RCU faces similar situations, i.e. raising softirq risks
> scheduler deadlock in rcu_read_unlock_special() -- but RCU's solution is to
> avoid raising the softirq and instead use irq_work.
Which is right.
> I was wondering, if we can rename __raise_softirq_irqoff() to
> raise_softirq_irqoff_no_wake() and call that from places where there is risk
> of scheduler related deadlocks. Then I think this can be used from Viktor's
> code. Let us discuss - what would happen if the softirq is raised, but
> ksoftirqd is not awakened for this latency notification path? Is this really
> an issue considering the softirq will execute during the next interrupt exit?
You'd get unbounded latency for processing the softirq and warnings on
going idle with softirqs pending.
I really don't see why we should/want to be using softirq here.
next prev parent reply other threads:[~2019-09-04 8:19 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20190903132602.3440-1-viktor.rosendahl@gmail.com>
[not found] ` <20190903132602.3440-2-viktor.rosendahl@gmail.com>
2019-09-04 4:00 ` [PATCH v5 1/4] ftrace: Implement fs notification for tracing_max_latency Joel Fernandes
2019-09-04 8:19 ` Peter Zijlstra [this message]
2019-09-04 13:42 ` Joel Fernandes
2019-09-04 18:17 ` Viktor Rosendahl
2019-09-04 10:21 ` Paul E. McKenney
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20190904081919.GA2349@hirez.programming.kicks-ass.net \
--to=peterz@infradead.org \
--cc=joel@joelfernandes.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-rt-users@vger.kernel.org \
--cc=mingo@redhat.com \
--cc=paulmck@kernel.org \
--cc=rostedt@goodmis.org \
--cc=viktor.rosendahl@gmail.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).