linux-kernel - Re: semaphore and mutex in current Linux kernel (3.2.2)

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Message-ID: <20120409184538.GE2430@linux.vnet.ibm.com>
Date:	Mon, 9 Apr 2012 11:45:39 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	"Chen, Dennis (SRDC SW)" <Dennis1.Chen@....com>
Cc:	Clemens Ladisch <clemens@...isch.de>,
	Ingo Molnar <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: semaphore and mutex in current Linux kernel (3.2.2)

On Fri, Apr 06, 2012 at 05:47:28PM +0000, Chen, Dennis (SRDC SW) wrote:
> On Fri, Apr 6, 2012 at 6:10 PM, Clemens Ladisch <clemens@...isch.de> wrote:
> > Chen, Dennis (SRDC SW) wrote:
> >> On Thu, Apr 5, 2012 at 10:15 PM, Clemens Ladisch <clemens@...isch.de> wrote:
> >>
> >> I guess this is related with RCU component, but I don't found the right place where the code
> >> Located yet.
> >
> > "On the internet, nobody can hear you being subtle."
> >
> > If some other process wants to run on the same CPU, needs_resched() is set.
> > (This might happen to make the cursor blink, for keyboard input, or when
> > somebody starts a rogue process like ps.)
> >
> 
> Hmm, I forget that in each timer interrupt, __rcu_pending() will be called, it will call
> set_need_resched() to set the TIF_NEED_RESCHED in some condition...
> The optimization of mutex work closely with rcu, so fantastic!

I must confess that you all lost me on this one.

There is a call to set_need_resched() in __rcu_pending(), which is
invoked when the current CPU has not yet responded to a non-preemptible
RCU grace period for some time.  However, in the common case where the
CPUs all respond in reasonable time, __rcu_pending() will never call
set_need_resched().

However, we really do not want to call set_need_resched() on every call
to __rcu_pending().  There is almost certainly a better solution to any
problem that might be solved by a per-jiffy call to set_need_resched().

So, what are you really trying to do?

							Thanx, Paul

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