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Date: Sat, 28 Mar 2015 17:27:23 +0530
From: viresh kumar <viresh.kumar@...aro.org>
To: Peter Zijlstra <peterz@...radead.org>
CC: Andrew Morton <akpm@...ux-foundation.org>,
Johannes Weiner <hannes@...xchg.org>,
Christoph Lameter <cl@...ux.com>,
Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
vinmenon@...eaurora.org, shashim@...eaurora.org,
Michal Hocko <mhocko@...e.cz>, Mel Gorman <mgorman@...e.de>,
dave@...olabs.net, Konstantin Khlebnikov <koct9i@...il.com>,
Linux Memory Management List <linux-mm@...ck.org>,
Suresh Siddha <suresh.b.siddha@...el.com>,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC] vmstat: Avoid waking up idle-cpu to service shepherd work
On 28 March 2015 at 15:23, Peter Zijlstra <peterz@...radead.org> wrote:
> Well, for one your patch is indeed disgusting.
Yeah, I agree :)
> But yes I'm aware Thomas
> wants to rewrite the timer thing. But Thomas is away for a little while
> and if this really needs to happen then it does.
Sometime back I was trying to use another bit from base pointer for
marking a timer as PINNED:
diff --git a/include/linux/timer.h b/include/linux/timer.h
index 8c5a197e1587..e7184f57449c 100644
--- a/include/linux/timer.h
+++ b/include/linux/timer.h
@@ -67,8 +67,9 @@ extern struct tvec_base boot_tvec_bases;
*/
#define TIMER_DEFERRABLE 0x1LU
#define TIMER_IRQSAFE 0x2LU
+#define TIMER_PINNED 0x4LU
-#define TIMER_FLAG_MASK 0x3LU
+#define TIMER_FLAG_MASK 0x7LU
And Fenguang's build-bot showed the problem (only) on blackfin [1].
config: make ARCH=blackfin allyesconfig
All error/warnings:
kernel/timer.c: In function 'init_timers':
>> kernel/timer.c:1683:2: error: call to '__compiletime_assert_1683'
>> declared with attribute error: BUILD_BUG_ON failed:
>> __alignof__(struct tvec_base) & TIMER_FLAG_MASK
So probably we need to make 'base' aligned to 8 bytes ?
So, what you are suggesting is something like this (untested):
diff --git a/include/linux/timer.h b/include/linux/timer.h
index 8c5a197e1587..68bf09d69352 100644
--- a/include/linux/timer.h
+++ b/include/linux/timer.h
@@ -67,8 +67,9 @@ extern struct tvec_base boot_tvec_bases;
*/
#define TIMER_DEFERRABLE 0x1LU
#define TIMER_IRQSAFE 0x2LU
+#define TIMER_RUNNING 0x4LU
-#define TIMER_FLAG_MASK 0x3LU
+#define TIMER_FLAG_MASK 0x7LU
#define __TIMER_INITIALIZER(_function, _expires, _data, _flags) { \
.entry = { .prev = TIMER_ENTRY_STATIC }, \
diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 2d3f5c504939..8f9efa64bd34 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -105,6 +105,21 @@ static inline unsigned int tbase_get_irqsafe(struct tvec_base *base)
return ((unsigned int)(unsigned long)base & TIMER_IRQSAFE);
}
+static inline unsigned int tbase_get_running(struct tvec_base *base)
+{
+ return ((unsigned int)(unsigned long)base & TIMER_RUNNING);
+}
+
+static inline unsigned int tbase_set_running(struct tvec_base *base)
+{
+ return ((unsigned int)(unsigned long)base | TIMER_RUNNING);
+}
+
+static inline unsigned int tbase_clear_running(struct tvec_base *base)
+{
+ return ((unsigned int)(unsigned long)base & ~TIMER_RUNNING);
+}
+
static inline struct tvec_base *tbase_get_base(struct tvec_base *base)
{
return ((struct tvec_base *)((unsigned long)base & ~TIMER_FLAG_MASK));
@@ -781,21 +796,12 @@ __mod_timer(struct timer_list *timer, unsigned long expires,
new_base = per_cpu(tvec_bases, cpu);
if (base != new_base) {
- /*
- * We are trying to schedule the timer on the local CPU.
- * However we can't change timer's base while it is running,
- * otherwise del_timer_sync() can't detect that the timer's
- * handler yet has not finished. This also guarantees that
- * the timer is serialized wrt itself.
- */
- if (likely(base->running_timer != timer)) {
- /* See the comment in lock_timer_base() */
- timer_set_base(timer, NULL);
- spin_unlock(&base->lock);
- base = new_base;
- spin_lock(&base->lock);
- timer_set_base(timer, base);
- }
+ /* See the comment in lock_timer_base() */
+ timer_set_base(timer, NULL);
+ spin_unlock(&base->lock);
+ base = new_base;
+ spin_lock(&base->lock);
+ timer_set_base(timer, base);
}
timer->expires = expires;
@@ -1016,7 +1022,7 @@ int try_to_del_timer_sync(struct timer_list *timer)
base = lock_timer_base(timer, &flags);
- if (base->running_timer != timer) {
+ if (tbase_get_running(timer->base)) {
timer_stats_timer_clear_start_info(timer);
ret = detach_if_pending(timer, base, true);
}
@@ -1202,6 +1208,7 @@ static inline void __run_timers(struct tvec_base *base)
timer_stats_account_timer(timer);
base->running_timer = timer;
+ tbase_set_running(timer->base);
detach_expired_timer(timer, base);
if (irqsafe) {
@@ -1216,6 +1223,7 @@ static inline void __run_timers(struct tvec_base *base)
}
}
base->running_timer = NULL;
+ tbase_clear_running(timer->base);
spin_unlock_irq(&base->lock);
}
------------x--------------------x----------------------
Right?
Now there are few issues I see here (Sorry if they are all imaginary):
- In case a timer re-arms itself from its handler and is migrated from CPU A to B, what
happens if the re-armed timer fires before the first handler finishes ? i.e. timer->fn()
hasn't finished running on CPU A and it has fired again on CPU B. Wouldn't this expose
us to a lot of other problems? It wouldn't be serialized to itself anymore ?
- Because the timer has migrated to another CPU, the locking in __run_timers()
needs to be fixed. And that will make it complicated ..
- __run_timer() doesn't lock bases of other CPUs, and it has to do it now..
- We probably need to take locks of both local CPU and the one to which timer migrated.
- Its possible now that there can be more than one running timer for a base, which wasn't
true earlier. Not sure if it will break something.
Thanks for your continuous support to reply to my (sometimes stupid) queries.
--
viresh
[1] https://lists.01.org/pipermail/kbuild-all/2014-April/003982.html
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