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Date: Tue, 17 Feb 2009 21:49:41 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
Nick Piggin <npiggin@...e.de>,
Jens Axboe <jens.axboe@...cle.com>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Ingo Molnar <mingo@...e.hu>,
Rusty Russell <rusty@...tcorp.com.au>,
Steven Rostedt <rostedt@...dmis.org>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH -v4] generic-ipi: remove kmalloc()
On Tue, 2009-02-17 at 21:16 +0100, Peter Zijlstra wrote:
> On Tue, 2009-02-17 at 21:11 +0100, Peter Zijlstra wrote:
> >
> > > As for the previous version. I am almost sure I missed something
> > > again, but do we really need both CSD_FLAG_WAIT and CSD_FLAG_LOCK
> > > flags? smp_call_function_many(wait => 1) can just wait for
> > > !CSD_FLAG_LOCK.
> >
> > I suppose we can.
>
> Ah, no, that would mean we cannot distinguish between the sync and async
> case in generic_exec_single().
Which can of course be solved like so, we could even drop the new .wait
argument to __smp_call_function_single() as CSD_FLAG_WAIT wasn't exposed
before and non of the current users use it.
---
Index: linux-2.6/kernel/smp.c
===================================================================
--- linux-2.6.orig/kernel/smp.c
+++ linux-2.6/kernel/smp.c
@@ -23,8 +23,7 @@ static struct {
};
enum {
- CSD_FLAG_WAIT = 0x01,
- CSD_FLAG_LOCK = 0x02,
+ CSD_FLAG_LOCK = 0x01,
};
struct call_function_data {
@@ -95,41 +94,21 @@ static int __cpuinit init_call_single_da
early_initcall(init_call_single_data);
/*
- * csd_wait/csd_complete are used for synchronous ipi calls
- */
-static void csd_wait_prepare(struct call_single_data *data)
-{
- data->flags |= CSD_FLAG_WAIT;
-}
-
-static void csd_complete(struct call_single_data *data)
-{
- if (data->flags & CSD_FLAG_WAIT) {
- /*
- * Serialize stores to data with the flag clear and wakeup.
- */
- smp_wmb();
- data->flags &= ~CSD_FLAG_WAIT;
- }
-}
-
-static void csd_wait(struct call_single_data *data)
-{
- while (data->flags & CSD_FLAG_WAIT)
- cpu_relax();
-}
-
-/*
* csd_lock/csd_unlock used to serialize access to per-cpu csd resources
*
* For non-synchronous ipi calls the csd can still be in use by the previous
* function call. For multi-cpu calls its even more interesting as we'll have
* to ensure no other cpu is observing our csd.
*/
-static void csd_lock(struct call_single_data *data)
+static void csd_lock_wait(struct call_single_data *data)
{
while (data->flags & CSD_FLAG_LOCK)
cpu_relax();
+}
+
+static void csd_lock(struct call_single_data *data)
+{
+ csd_lock_wait(data);
data->flags = CSD_FLAG_LOCK;
}
@@ -147,11 +126,12 @@ static void csd_unlock(struct call_singl
* Insert a previously allocated call_single_data element for execution
* on the given CPU. data must already have ->func, ->info, and ->flags set.
*/
-static void generic_exec_single(int cpu, struct call_single_data *data)
+static
+void generic_exec_single(int cpu, struct call_single_data *data, int wait)
{
struct call_single_queue *dst = &per_cpu(call_single_queue, cpu);
- int wait = data->flags & CSD_FLAG_WAIT, ipi;
unsigned long flags;
+ int ipi;
spin_lock_irqsave(&dst->lock, flags);
ipi = list_empty(&dst->list);
@@ -174,7 +154,7 @@ static void generic_exec_single(int cpu,
arch_send_call_function_single_ipi(cpu);
if (wait)
- csd_wait(data);
+ csd_lock_wait(data);
}
/*
@@ -222,7 +202,6 @@ void generic_smp_call_function_interrupt
if (refs)
continue;
- csd_complete(&data->csd);
csd_unlock(&data->csd);
}
@@ -260,9 +239,6 @@ void generic_smp_call_function_single_in
data->func(data->info);
- if (data_flags & CSD_FLAG_WAIT)
- csd_complete(data);
-
/*
* Unlocked CSDs are valid through generic_exec_single()
*/
@@ -303,36 +279,16 @@ int smp_call_function_single(int cpu, vo
func(info);
local_irq_restore(flags);
} else if ((unsigned)cpu < nr_cpu_ids && cpu_online(cpu)) {
- struct call_single_data *data;
+ struct call_single_data *data = &d;
- if (!wait) {
- /*
- * We are calling a function on a single CPU
- * and we are not going to wait for it to finish.
- * We use a per cpu data to pass the information to
- * that CPU. Since all callers of this code will
- * use the same data, we must synchronize the
- * callers to prevent a new caller from corrupting
- * the data before the callee can access it.
- *
- * The CSD_FLAG_LOCK is used to let us know when
- * the IPI handler is done with the data.
- * The first caller will set it, and the callee
- * will clear it. The next caller must wait for
- * it to clear before we set it again. This
- * will make sure the callee is done with the
- * data before a new caller will use it.
- */
+ if (!wait)
data = &per_cpu(csd_data, me);
- csd_lock(data);
- } else {
- data = &d;
- csd_wait_prepare(data);
- }
+
+ csd_lock(data);
data->func = func;
data->info = info;
- generic_exec_single(cpu, data);
+ generic_exec_single(cpu, data, wait);
} else {
err = -ENXIO; /* CPU not online */
}
@@ -352,12 +308,16 @@ EXPORT_SYMBOL(smp_call_function_single);
* instance.
*
*/
-void __smp_call_function_single(int cpu, struct call_single_data *data)
+void __smp_call_function_single(int cpu, struct call_single_data *data,
+ int wait)
{
+ if (wait)
+ csd_lock(data);
+
/* Can deadlock when called with interrupts disabled */
- WARN_ON((data->flags & CSD_FLAG_WAIT) && irqs_disabled());
+ WARN_ON(wait && irqs_disabled());
- generic_exec_single(cpu, data);
+ generic_exec_single(cpu, data, wait);
}
/* FIXME: Shim for archs using old arch_send_call_function_ipi API. */
@@ -415,9 +375,6 @@ void smp_call_function_many(const struct
csd_lock(&data->csd);
spin_lock_irqsave(&data->lock, flags);
- if (wait)
- csd_wait_prepare(&data->csd);
-
data->csd.func = func;
data->csd.info = info;
cpumask_and(data->cpumask, mask, cpu_online_mask);
@@ -446,7 +403,7 @@ void smp_call_function_many(const struct
/* optionally wait for the CPUs to complete */
if (wait)
- csd_wait(&data->csd);
+ csd_lock_wait(&data->csd);
}
EXPORT_SYMBOL(smp_call_function_many);
Index: linux-2.6/block/blk-softirq.c
===================================================================
--- linux-2.6.orig/block/blk-softirq.c
+++ linux-2.6/block/blk-softirq.c
@@ -64,7 +64,7 @@ static int raise_blk_irq(int cpu, struct
data->info = rq;
data->flags = 0;
- __smp_call_function_single(cpu, data);
+ __smp_call_function_single(cpu, data, 0);
return 0;
}
Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -1120,7 +1120,7 @@ static void hrtick_start(struct rq *rq,
if (rq == this_rq()) {
hrtimer_restart(timer);
} else if (!rq->hrtick_csd_pending) {
- __smp_call_function_single(cpu_of(rq), &rq->hrtick_csd);
+ __smp_call_function_single(cpu_of(rq), &rq->hrtick_csd, 0);
rq->hrtick_csd_pending = 1;
}
}
Index: linux-2.6/kernel/softirq.c
===================================================================
--- linux-2.6.orig/kernel/softirq.c
+++ linux-2.6/kernel/softirq.c
@@ -518,7 +518,7 @@ static int __try_remote_softirq(struct c
cp->flags = 0;
cp->priv = softirq;
- __smp_call_function_single(cpu, cp);
+ __smp_call_function_single(cpu, cp, 0);
return 0;
}
return 1;
Index: linux-2.6/include/linux/smp.h
===================================================================
--- linux-2.6.orig/include/linux/smp.h
+++ linux-2.6/include/linux/smp.h
@@ -82,7 +82,8 @@ smp_call_function_mask(cpumask_t mask, v
return 0;
}
-void __smp_call_function_single(int cpuid, struct call_single_data *data);
+void __smp_call_function_single(int cpuid, struct call_single_data *data,
+ int wait);
/*
* Generic and arch helpers
--
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