On 24 April 2013 16:52, Viresh Kumar [off-list ref] wrote:

On 9 April 2013 20:22, Viresh Kumar [off-list ref] wrote:

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[Steven replied to a personal Ping!!, including everybody again]

On 9 April 2013 19:25, Steven Rostedt [off-list ref] wrote:

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On Tue, 2013-04-09 at 14:05 +0530, Viresh Kumar wrote:

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Ping!!

Remind me again. What problem are you trying to solve?

I was trying to migrate a running timer which arms itself, so that we don't
keep a cpu busy just for servicing this timer.

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On 20 March 2013 20:43, Viresh Kumar [off-list ref] wrote:

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Hi Steven/Thomas,

I came back to this patch after completing some other stuff and posting
wq part of this patchset separately.

I got your point and understand how this would fail.

@Thomas: I need your opinion first. Do you like this concept of migrating
running timer or not? Or you see some basic problem with this concept?

I'll let Thomas answer this, but to me, this sounds really racy.

Sure.

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If no (i.e. i can go ahead with another version), then i have some solution to
fix earlier problems reported by Steven:

The problem lies with del_timer_sync() which just checks
base->running_timer != timer to check if timer is currently running or not.

What if we add another variable in struct timer_list, that will store if we are
running timer callback or not. And so, before we call callback in timer core,
we will set this variable and will reset it after finishing callback.

del_timer_sync() will have something like:

if (base->running_timer != timer)
    remove timer and return;

For example, this didn't fix the issue. You removed the timer when it
was still running, because base->running_timer did not equal timer.

You are correct and i was stupid. I wanted to write this instead:

del_timer_sync() will have something like:

if (base->running_timer != timer)
    if (timer->running_callback)
        go back to its loop...
    else
        remove timer and return;

i.e. if we aren't running on our base cpu, just check if our callback is
executing somewhere else due to migration.

Ping!!

Ping!!