On Tue, 13 Sep 2016, Peter Zijlstra wrote:
> Hi all,
>
> While grepping for PREEMPT_VOLUNTARY I ran into dm_bufio_cond_resched()
> and wondered WTH it was about.
cond_resched() calls _cond_resched() even if when we have a preemptive
kernel - with preemptive kernel, calling cond_resched is pointless because
rescheduling is done peemtively.
So, I added that dm_bufio_cond_resched(), that does nothing on peemptive
kernels (and also on PREEMPT_NONE kernels where the user doesn't care
about latency).
What is the reason why cond_resched() tests for rescheduling with
preemptive kernel? Why should I use cond_resched() in that case?
Mikulas
> Is there anything wrong with the below patch?
>
> ---
> diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
> index 8625040bae92..125aedc3875f 100644
> --- a/drivers/md/dm-bufio.c
> +++ b/drivers/md/dm-bufio.c
> @@ -191,19 +191,6 @@ static void dm_bufio_unlock(struct dm_bufio_client *c)
> mutex_unlock(&c->lock);
> }
>
> -/*
> - * FIXME Move to sched.h?
> - */
> -#ifdef CONFIG_PREEMPT_VOLUNTARY
> -# define dm_bufio_cond_resched() \
> -do { \
> - if (unlikely(need_resched())) \
> - _cond_resched(); \
> -} while (0)
> -#else
> -# define dm_bufio_cond_resched() do { } while (0)
> -#endif
> -
> /*----------------------------------------------------------------*/
>
> /*
> @@ -741,7 +728,7 @@ static void __flush_write_list(struct list_head *write_list)
> list_entry(write_list->next, struct dm_buffer, write_list);
> list_del(&b->write_list);
> submit_io(b, WRITE, b->block, write_endio);
> - dm_bufio_cond_resched();
> + cond_resched();
> }
> blk_finish_plug(&plug);
> }
> @@ -780,7 +767,7 @@ static struct dm_buffer *__get_unclaimed_buffer(struct dm_bufio_client *c)
> __unlink_buffer(b);
> return b;
> }
> - dm_bufio_cond_resched();
> + cond_resched();
> }
>
> list_for_each_entry_reverse(b, &c->lru[LIST_DIRTY], lru_list) {
> @@ -791,7 +778,7 @@ static struct dm_buffer *__get_unclaimed_buffer(struct dm_bufio_client *c)
> __unlink_buffer(b);
> return b;
> }
> - dm_bufio_cond_resched();
> + cond_resched();
> }
>
> return NULL;
> @@ -923,7 +910,7 @@ static void __write_dirty_buffers_async(struct dm_bufio_client *c, int no_wait,
> return;
>
> __write_dirty_buffer(b, write_list);
> - dm_bufio_cond_resched();
> + cond_resched();
> }
> }
>
> @@ -973,7 +960,7 @@ static void __check_watermark(struct dm_bufio_client *c,
> return;
>
> __free_buffer_wake(b);
> - dm_bufio_cond_resched();
> + cond_resched();
> }
>
> if (c->n_buffers[LIST_DIRTY] > threshold_buffers)
> @@ -1170,7 +1157,7 @@ void dm_bufio_prefetch(struct dm_bufio_client *c,
> submit_io(b, READ, b->block, read_endio);
> dm_bufio_release(b);
>
> - dm_bufio_cond_resched();
> + cond_resched();
>
> if (!n_blocks)
> goto flush_plug;
> @@ -1291,7 +1278,7 @@ int dm_bufio_write_dirty_buffers(struct dm_bufio_client *c)
> !test_bit(B_WRITING, &b->state))
> __relink_lru(b, LIST_CLEAN);
>
> - dm_bufio_cond_resched();
> + cond_resched();
>
> /*
> * If we dropped the lock, the list is no longer consistent,
> @@ -1574,7 +1561,7 @@ static unsigned long __scan(struct dm_bufio_client *c, unsigned long nr_to_scan,
> freed++;
> if (!--nr_to_scan || ((count - freed) <= retain_target))
> return freed;
> - dm_bufio_cond_resched();
> + cond_resched();
> }
> }
> return freed;
> @@ -1808,7 +1795,7 @@ static void __evict_old_buffers(struct dm_bufio_client *c, unsigned long age_hz)
> if (__try_evict_buffer(b, 0))
> count--;
>
> - dm_bufio_cond_resched();
> + cond_resched();
> }
>
> dm_bufio_unlock(c);
>
--
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