On Tue, 3 Feb 2009 17:58:13 +0100
Fr__d__ric Weisbecker <fweisbec@xxxxxxxxx> wrote:
> 2009/2/3 Ingo Molnar <mingo@xxxxxxx>:
> >
> > * akpm@xxxxxxxxxxxxxxxxxxxx <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> >> ------------------------------------------------------
> >> Subject: work_on_cpu(): rewrite it to create a kernel thread on demand
> >> From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> >>
> >> The various implemetnations and proposed implemetnations of work_on_cpu()
> >> are vulnerable to various deadlocks because they all used queues of some
> >> form.
> >>
> >> Unrelated pieces of kernel code thus gained dependencies wherein if one
> >> work_on_cpu() caller holds a lock which some other work_on_cpu() callback
> >> also takes, the kernel could rarely deadlock.
> >>
> >> Fix this by creating a short-lived kernel thread for each work_on_cpu()
> >> invokation.
> >>
> >> This is not terribly fast, but the only current caller of work_on_cpu() is
> >> pci_call_probe().
> >
> > hm, it's quite ugly as well
No it isn't.
It's no less ugly than the current code.
It's less buggy than the current code.
>, and wasteful with resources.
The current code consumes about 10kbytes per cpu and one kernel thread
per cpu. This code fixes that.
(ie: since when did you guys care about consuming resources?)
> Sorry I don't see the patch but only the changelog.
> So perhaps my answer will be a bit out of sync.
>
> But if pci_call_probe() is the only caller, so it is supposed to be
> called only on boot.
> Perhaps the work_on_cpu thread can be killed after boot up and then
> become a thread created
> on the fly after that if needed....
>
> Or perhaps it's too much complex.....
Series of four patches:
- switch cstate.c frmo work_on_cpu to smp_call_function_single()
- ditto acpi-cpufreq.c
- ditto mce_amd_64.c
The final work_on_cpu() caller is pci_call_probe(). I'd like to find a
way of removing that callsite as well, so we can finally remove this
turkey but for now, just fix the bugs in it:
From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
The various implementations and proposed implementations of work_on_cpu()
are vulnerable to various deadlocks because they all use queues of some
form.
Unrelated pieces of kernel code thus gained dependencies wherein if one
work_on_cpu() caller holds a lock which some other work_on_cpu() callback
also takes, the kernel could rarely deadlock.
Also, the present work_on_cpu() implementation creates yet another kernel
thread per CPU.
Fix this by creating a short-lived kernel thread for each work_on_cpu()
invokation.
This is not terribly fast, but the only current caller of work_on_cpu() is
pci_call_probe().
It would be nice to find some other way of doing the node-local
allocations in the PCI probe code so that we can zap work_on_cpu()
altogether. The code there is rather nasty. I can't think of anything
simple at this time...
Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/workqueue.c | 36 +++++++++++++++++++-----------------
1 file changed, 19 insertions(+), 17 deletions(-)
diff -puN kernel/workqueue.c~work_on_cpu-rewrite-it-to-create-a-kernel-thread-on-demand kernel/workqueue.c
--- a/kernel/workqueue.c~work_on_cpu-rewrite-it-to-create-a-kernel-thread-on-demand
+++ a/kernel/workqueue.c
@@ -971,20 +971,20 @@ undo:
}
#ifdef CONFIG_SMP
-static struct workqueue_struct *work_on_cpu_wq __read_mostly;
struct work_for_cpu {
- struct work_struct work;
+ struct completion completion;
long (*fn)(void *);
void *arg;
long ret;
};
-static void do_work_for_cpu(struct work_struct *w)
+static int do_work_for_cpu(void *_wfc)
{
- struct work_for_cpu *wfc = container_of(w, struct work_for_cpu, work);
-
+ struct work_for_cpu *wfc = _wfc;
wfc->ret = wfc->fn(wfc->arg);
+ complete(&wfc->completion);
+ return 0;
}
/**
@@ -995,17 +995,23 @@ static void do_work_for_cpu(struct work_
*
* This will return the value @fn returns.
* It is up to the caller to ensure that the cpu doesn't go offline.
+ * The caller must not hold any locks which would prevent @fn from completing.
*/
long work_on_cpu(unsigned int cpu, long (*fn)(void *), void *arg)
{
- struct work_for_cpu wfc;
-
- INIT_WORK(&wfc.work, do_work_for_cpu);
- wfc.fn = fn;
- wfc.arg = arg;
- queue_work_on(cpu, work_on_cpu_wq, &wfc.work);
- flush_work(&wfc.work);
-
+ struct task_struct *sub_thread;
+ struct work_for_cpu wfc = {
+ .completion = COMPLETION_INITIALIZER_ONSTACK(wfc.completion),
+ .fn = fn,
+ .arg = arg,
+ };
+
+ sub_thread = kthread_create(do_work_for_cpu, &wfc, "work_for_cpu");
+ if (IS_ERR(sub_thread))
+ return PTR_ERR(sub_thread);
+ kthread_bind(sub_thread, cpu);
+ wake_up_process(sub_thread);
+ wait_for_completion(&wfc.completion);
return wfc.ret;
}
EXPORT_SYMBOL_GPL(work_on_cpu);
@@ -1021,8 +1027,4 @@ void __init init_workqueues(void)
hotcpu_notifier(workqueue_cpu_callback, 0);
keventd_wq = create_workqueue("events");
BUG_ON(!keventd_wq);
-#ifdef CONFIG_SMP
- work_on_cpu_wq = create_workqueue("work_on_cpu");
- BUG_ON(!work_on_cpu_wq);
-#endif
}
_
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