Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759673AbZFBHys (ORCPT ); Tue, 2 Jun 2009 03:54:48 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752414AbZFBHyj (ORCPT ); Tue, 2 Jun 2009 03:54:39 -0400 Received: from 124x34x33x190.ap124.ftth.ucom.ne.jp ([124.34.33.190]:34083 "EHLO master.linux-sh.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757545AbZFBHyi (ORCPT ); Tue, 2 Jun 2009 03:54:38 -0400 Date: Tue, 2 Jun 2009 16:54:09 +0900 From: Paul Mundt To: Peter Zijlstra Cc: Ingo Molnar , Thomas Gleixner , Daniel Walker , Linus Walleij , Andrew Victor , Haavard Skinnemoen , Andrew Morton , John Stultz , linux-arm-kernel@lists.arm.linux.org.uk, linux-sh@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] sched: sched_clock() clocksource handling. Message-ID: <20090602075409.GA19294@linux-sh.org> Mail-Followup-To: Paul Mundt , Peter Zijlstra , Ingo Molnar , Thomas Gleixner , Daniel Walker , Linus Walleij , Andrew Victor , Haavard Skinnemoen , Andrew Morton , John Stultz , linux-arm-kernel@lists.arm.linux.org.uk, linux-sh@vger.kernel.org, linux-kernel@vger.kernel.org References: <20090602071718.GA17710@linux-sh.org> <1243927502.23657.5619.camel@twins> <20090602073515.GB17710@linux-sh.org> <1243928495.23657.5642.camel@twins> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1243928495.23657.5642.camel@twins> User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5488 Lines: 166 On Tue, Jun 02, 2009 at 09:41:35AM +0200, Peter Zijlstra wrote: > On Tue, 2009-06-02 at 16:35 +0900, Paul Mundt wrote: > > > > We already do via select_clocksource(), if we are unregistering the > > current one then a new one with the flag set is selected. Before that, > > the override is likewise given preference, and we fall back on jiffies if > > there is nothing else. I suppose we could try and find the "best" one, > > but I think the override and manual clocksource selection should be fine > > for this. > > Ah, ok. So unregister calls select_clocksource again? That does leave us > a small window with jiffies, but I guess that's ok. > A synchronize_rcu() would fix that up, but I think a small window with jiffies is less painful than sorting out RCU ordering and synchronization for a corner case of a corner case ;-) > > Now that you mention it though, the sched_clocksource() assignment within > > select_clocksource() happens underneath the clocksource_lock, but is not > > using rcu_assign_pointer(). > > Right, that would want fixing indeed. > > > If the assignment there needs to use > > rcu_assign_pointer() then presumably all of the unlock paths that do > > select_clocksource() will have to synchronize_rcu()? > > No, you only have to do sync_rcu() when stuff that could have referenced > is going away and you cannot use call_rcu(). > > So when selecting a new clocksource, you don't need synchonization > because stuff doesn't go away (I think :-) Ok, that keeps things more simplified then. How does this look? --- include/linux/clocksource.h | 4 +++- kernel/sched_clock.c | 13 +++++++++++-- kernel/time/clocksource.c | 19 +++++++++++++++++++ kernel/time/jiffies.c | 2 +- 4 files changed, 34 insertions(+), 4 deletions(-) diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h index c56457c..2109940 100644 --- a/include/linux/clocksource.h +++ b/include/linux/clocksource.h @@ -202,7 +202,8 @@ struct clocksource { #endif }; -extern struct clocksource *clock; /* current clocksource */ +extern struct clocksource *clock; /* current clocksource */ +extern struct clocksource *sched_clocksource; /* sched_clock() clocksource */ /* * Clock source flags bits:: @@ -212,6 +213,7 @@ extern struct clocksource *clock; /* current clocksource */ #define CLOCK_SOURCE_WATCHDOG 0x10 #define CLOCK_SOURCE_VALID_FOR_HRES 0x20 +#define CLOCK_SOURCE_USE_FOR_SCHED_CLOCK 0x40 /* simplify initialization of mask field */ #define CLOCKSOURCE_MASK(bits) (cycle_t)((bits) < 64 ? ((1ULL<<(bits))-1) : -1) diff --git a/kernel/sched_clock.c b/kernel/sched_clock.c index e1d16c9..b51d48d 100644 --- a/kernel/sched_clock.c +++ b/kernel/sched_clock.c @@ -30,6 +30,8 @@ #include #include #include +#include +#include /* * Scheduler clock - returns current time in nanosec units. @@ -38,8 +40,15 @@ */ unsigned long long __attribute__((weak)) sched_clock(void) { - return (unsigned long long)(jiffies - INITIAL_JIFFIES) - * (NSEC_PER_SEC / HZ); + unsigned long long time; + struct clocksource *clock; + + rcu_read_lock(); + clock = rcu_dereference(sched_clocksource); + time = cyc2ns(clock, clocksource_read(clock)); + rcu_read_unlock(); + + return time; } static __read_mostly int sched_clock_running; diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index 80189f6..f7243f2 100644 --- a/kernel/time/clocksource.c +++ b/kernel/time/clocksource.c @@ -25,6 +25,7 @@ */ #include +#include #include #include #include @@ -109,6 +110,7 @@ EXPORT_SYMBOL(timecounter_cyc2time); /* XXX - Would like a better way for initializing curr_clocksource */ extern struct clocksource clocksource_jiffies; +struct clocksource *sched_clocksource = &clocksource_jiffies; /*[Clocksource internal variables]--------- * curr_clocksource: @@ -362,6 +364,9 @@ static struct clocksource *select_clocksource(void) if (next == curr_clocksource) return NULL; + if (next->flags & CLOCK_SOURCE_USE_FOR_SCHED_CLOCK) + rcu_assign_pointer(sched_clocksource, next); + return next; } @@ -440,7 +445,21 @@ void clocksource_unregister(struct clocksource *cs) list_del(&cs->list); if (clocksource_override == cs) clocksource_override = NULL; + next_clocksource = select_clocksource(); + + /* + * If select_clocksource() fails to find another suitable + * clocksource for sched_clocksource and we are unregistering + * it, switch back to jiffies. + */ + if (sched_clocksource == cs) { + rcu_assign_pointer(sched_clocksource, &clocksource_jiffies); + spin_unlock_irqrestore(&clocksource_lock, flags); + synchronize_rcu(); + return; + } + spin_unlock_irqrestore(&clocksource_lock, flags); } diff --git a/kernel/time/jiffies.c b/kernel/time/jiffies.c index c3f6c30..727d881 100644 --- a/kernel/time/jiffies.c +++ b/kernel/time/jiffies.c @@ -52,7 +52,7 @@ static cycle_t jiffies_read(struct clocksource *cs) { - return (cycle_t) jiffies; + return (cycle_t) (jiffies - INITIAL_JIFFIES); } struct clocksource clocksource_jiffies = { -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/