Re: [RFC][PATCH] dm: Remove dm_bufio_cond_resched()

[Mikulas Patocka <mpatocka@redhat.com>]

On Mon, 19 Sep 2016, Peter Zijlstra wrote:

> On Tue, Sep 13, 2016 at 09:39:59AM -0400, Mike Snitzer wrote:
> > So I'm not sure how this dm-bufio local cond_resched() wrapper still got
> > in... happy to take your patch.
> > 
> > Please respond with whatever SOB you'd like applied to the patch header.
> 
> Sorry, for the delay, here goes.

Why not change it to might_sleep()? - that would be almost equivalent to 
the code that was there before (i.e. reschedule only if 
CONFIG_PREEMPT_VOLUNTARY is set).

If we call the cond_resched() function in tight loops such as walking all 
buffers in a list, there may be performance penalty due to the call, so 
the call should be done only if it is really needed (i.e. in 
CONFIG_PREEMPT_VOLUNTARY case).

Mikulas

> ---
> Subject: dm: Remove dm_bufio_cond_resched()
> From: Peter Zijlstra <peterz@infradead.org>
> Date: Tue, 13 Sep 2016 10:45:20 +0200
> 
> Remove pointless local wrappery. Use cond_resched() like everybody else.
> 
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Mikulas Patocka <mpatocka@redhat.com>
> Cc: Mike Snitzer <snitzer@redhat.com>
> Cc: Alasdair Kergon <agk@redhat.com>
> Acked-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  drivers/md/dm-bufio.c |   31 +++++++++----------------------
>  1 file changed, 9 insertions(+), 22 deletions(-)
> 
> --- a/drivers/md/dm-bufio.c
> +++ b/drivers/md/dm-bufio.c
> @@ -191,19 +191,6 @@ static void dm_bufio_unlock(struct dm_bu
>  	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 li
>  			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
>  			__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
>  			__unlink_buffer(b);
>  			return b;
>  		}
> -		dm_bufio_cond_resched();
> +		cond_resched();
>  	}
>  
>  	return NULL;
> @@ -923,7 +910,7 @@ static void __write_dirty_buffers_async(
>  			return;
>  
>  		__write_dirty_buffer(b, write_list);
> -		dm_bufio_cond_resched();
> +		cond_resched();
>  	}
>  }
>  
> @@ -973,7 +960,7 @@ static void __check_watermark(struct dm_
>  			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_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
>  		    !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_bu
>  				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 d
>  		if (__try_evict_buffer(b, 0))
>  			count--;
>  
> -		dm_bufio_cond_resched();
> +		cond_resched();
>  	}
>  
>  	dm_bufio_unlock(c);
>