Re: [PATCH] md/r5cache: run_no_space_stripes() when R5C_LOG_CRITICAL == 0

[Shaohua Li <shli@kernel.org>]

On Wed, Nov 23, 2016 at 12:17:45PM -0800, Song Liu wrote:
> With writeback cache, we define log space critical as
> 
>    free_space < 2 * reclaim_required_space
> 
> So the deassert of R5C_LOG_CRITICAL could happen when
>   1. free_space increases
>   2. reclaim_required_space decreases
> 
> Currently, run_no_space_stripes() is called when 1 happens, but
> not (always) when 2 happens.
> 
> With this patch, run_no_space_stripes() is call when
> R5C_LOG_CRITICAL is cleared.
> 
> Signed-off-by: Song Liu <songliubraving@fb.com>
> ---
>  drivers/md/raid5-cache.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/md/raid5-cache.c b/drivers/md/raid5-cache.c
> index 8cb79fc..0e24aab 100644
> --- a/drivers/md/raid5-cache.c
> +++ b/drivers/md/raid5-cache.c
> @@ -370,6 +370,7 @@ static inline void r5c_update_log_state(struct r5l_log *log)
>  	struct r5conf *conf = log->rdev->mddev->private;
>  	sector_t free_space;
>  	sector_t reclaim_space;
> +	bool wake_reclaim = false;
>  
>  	if (!r5c_is_writeback(log))
>  		return;
> @@ -379,12 +380,18 @@ static inline void r5c_update_log_state(struct r5l_log *log)
>  	reclaim_space = r5c_log_required_to_flush_cache(conf);
>  	if (free_space < 2 * reclaim_space)
>  		set_bit(R5C_LOG_CRITICAL, &conf->cache_state);
> -	else
> +	else {
> +		if (test_bit(R5C_LOG_CRITICAL, &conf->cache_state))
> +			wake_reclaim = true;
>  		clear_bit(R5C_LOG_CRITICAL, &conf->cache_state);
> +	}

This doesn't look correct. shouldn't we clear the bit and do the wakeup right
after stripe_in_journal_count is decreased? Actually all these clear_bit looks
wrong. They should be cleared right after the condition met. In current
implementation, if no new stripe comes, r5c_update_log_state will not get
called.

>  	if (free_space < 3 * reclaim_space)
>  		set_bit(R5C_LOG_TIGHT, &conf->cache_state);
>  	else
>  		clear_bit(R5C_LOG_TIGHT, &conf->cache_state);
> +
> +	if (wake_reclaim)
> +		r5l_wake_reclaim(log, 0);
>  }
>  
>  /*
> @@ -1348,6 +1355,10 @@ static void r5c_do_reclaim(struct r5conf *conf)
>  		spin_unlock(&conf->device_lock);
>  		spin_unlock_irqrestore(&log->stripe_in_journal_lock, flags);
>  	}
> +
> +	if (!test_bit(R5C_LOG_CRITICAL, &conf->cache_state))
> +		r5l_run_no_space_stripes(log);
> +

Why this? r5l_do_reclaim will call r5l_run_no_space_stripes.

Thanks,
Shaohua