Re: [PATCH v2] block: document blk-plug

[Shaohua Li]

On Tue, 2011-08-30 at 13:21 +0800, Suresh Jayaraman wrote:
> On 08/30/2011 03:18 AM, Andrew Morton wrote:
> > On Mon, 29 Aug 2011 16:58:21 +0530
> > Suresh Jayaraman <sjayaraman@xxxxxxx> wrote:
> > 
> >> --- a/include/linux/blkdev.h
> >> +++ b/include/linux/blkdev.h
> >> @@ -863,17 +863,23 @@ struct request_queue *blk_alloc_queue_node(gfp_t, int);
> >>  extern void blk_put_queue(struct request_queue *);
> >>  
> >>  /*
> >> + * blk_plug allows to build up a queue of related requests by holding the I/O
> >> + * fragments for a short period. This allows merging of sequential requests
> >> + * into single larger request. As the requests are moved from per-task list to
> >> + * the device's request_queue in a batch, this results in improved
> >> + * scalability as the lock contention for request_queue lock is reduced.
> >> + *
> >>   * Note: Code in between changing the blk_plug list/cb_list or element of such
> >>   * lists is preemptable, but such code can't do sleep (or be very careful),
> >>   * otherwise data is corrupted. For details, please check schedule() where
> >>   * blk_schedule_flush_plug() is called.
> > 
> > What does the older part of this comment mean?  If a code section is
> > preemptible then it *will* sleep.  That's what preemption does.
> > 
> 
> From what I can understand, we don't need to explicitly disable preemption
> when modifying the blk_plug->list because interrupts are disabled when we
> are there.
> 
> void blk_flush_plug_list(struct blk_plug *plug, bool from_schedule)
> {
> 
> ..
> 
>         /*
>          * Save and disable interrupts here, to avoid doing it for every
>          * queue lock we have to take.
>          */
>         local_irq_save(flags);
>         while (!list_empty(&list)) {
>                 rq = list_entry_rq(list.next);
>                 list_del_init(&rq->queuelist);
>                 BUG_ON(!rq->q);
>                 if (rq->q != q) {
>                         /*
>                          * This drops the queue lock
>                          */
>                         if (q)
>                                 queue_unplugged(q, depth, from_schedule);
>                         q = rq->q;
>                         depth = 0;
>                         spin_lock(q->queue_lock);
>                 }
> 
> 
> ..
> 
> }
> 
> When blk_flush_plug_list() is called from schedule() via
> blk_schedule_flush_plug() we must be very careful to not cause
> need_resched set and thus result in a preemption check?
> 
> Does that what your comment intend to mean? Shaohua?
the code adding request to the plug list and doing merge doesn't disable
preempt. That is ok because blk_schedule_flush_plug() only flush the
list when the task truly enters sleep (setting task->state non-running
and call schedule()). That's why I mean the code can be preempted but
can't do sleep.

Thanks,
Shaohua

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