LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Peter Zijlstra <peterz@infradead.org>
To: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	Linaro Kernel Mailman List <linaro-kernel@lists.linaro.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/2] hrtimer: update '->active_bases' before calling hrtimer_force_reprogram()
Date: Thu, 2 Apr 2015 16:16:33 +0200	[thread overview]
Message-ID: <20150402141633.GC23123@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <CAKohpok=DnS+h-YCF8+=xeK9_4i9S=x+sKan9SWJYVEYiA99vA@mail.gmail.com>

On Thu, Apr 02, 2015 at 07:23:31PM +0530, Viresh Kumar wrote:
> On 2 April 2015 at 19:17, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Thu, Apr 02, 2015 at 04:21:21PM +0530, Viresh Kumar wrote:
> >> 'active_bases' indicates which clock-base have active timers. While it
> >> is updated (almost) correctly, it is hardly used. Next commit will start
> >> using it to make code more efficient, but before that we need to fix a
> >> problem.
> >>
> >> While removing hrtimers, in __remove_hrtimer():
> >> - We first remove the hrtimer from the queue.
> >> - Then reprogram clockevent device if required
> >>   (hrtimer_force_reprogram()).
> >> - And then finally clear 'active_bases', if no more timers are pending
> >>   on the current clock base (from which we are removing the hrtimer).
> >>
> >> hrtimer_force_reprogram() needs to loop over all active clock bases to
> >> find the next expiry event, and while doing so it will use
> >> 'active_bases' (after next commit). And it will find the current base
> >> active, as we haven't cleared it until now, even if current clock base
> >> has no more hrtimers queued.
> >>
> >> To fix this issue, clear active_bases before calling
> >> hrtimer_force_reprogram().
> >
> > This is a small inefficiency right? Not an actual bug or anything.
> 
> So, what's explained in this patch is a BUG, which isn't harming us today.

So then I'm not seeing how its a bug. Sure __hrtimer_get_next_event()
will iterate all the bases again, and it will not skip the just empty
one. But I don't see how that is anything but an inefficiency. By virtue
of the base being empty it cannot find an event there, so its a
pointless check.

What am I missing?

  reply	other threads:[~2015-04-02 14:16 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-04-02 10:51 [PATCH 0/2] hrtimer: Only iterate over active bases Viresh Kumar
2015-04-02 10:51 ` [PATCH 1/2] hrtimer: update '->active_bases' before calling hrtimer_force_reprogram() Viresh Kumar
2015-04-02 13:47   ` Peter Zijlstra
2015-04-02 13:53     ` Viresh Kumar
2015-04-02 14:16       ` Peter Zijlstra [this message]
2015-04-02 14:23         ` Viresh Kumar
2015-04-02 10:51 ` [PATCH 2/2] hrtimer: create for_each_active_base() to iterate over active clock-bases Viresh Kumar
2015-04-02 13:45   ` Peter Zijlstra
2015-04-03  5:42     ` viresh kumar

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=20150402141633.GC23123@twins.programming.kicks-ass.net \
    --to=peterz@infradead.org \
    --cc=linaro-kernel@lists.linaro.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mingo@redhat.com \
    --cc=tglx@linutronix.de \
    --cc=viresh.kumar@linaro.org \
    --subject='Re: [PATCH 1/2] hrtimer: update '\''->active_bases'\'' before calling hrtimer_force_reprogram()' \
    /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

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).