linux-kernel - Re: [PATCH 1/1] ext4: fix lazy initialization next schedule time computation in more granular unit

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Date:   Thu, 14 Oct 2021 11:56:57 +0200
From:   Jan Kara <jack@...e.cz>
To:     Shaoying Xu <shaoyi@...zon.com>
Cc:     tytso@....edu, adilger.kernel@...ger.ca,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        fllinden@...zon.com, benh@...zon.com
Subject: Re: [PATCH 1/1] ext4: fix lazy initialization next schedule time
 computation in more granular unit

On Tue 17-08-21 22:56:54, Shaoying Xu wrote:
> Ext4 file system has default lazy inode table initialization setup once
> it is mounted. However, it has issue on computing the next schedule time
> that makes the timeout same amount in jiffies but different real time in
> secs if with various HZ values. Therefore, fix by measuring the current
> time in a more granular unit nanoseconds and make the next schedule time
> independent of the HZ value.
> 
> Fixes: bfff68738f1c ("ext4: add support for lazy inode table initialization")
> Signed-off-by: Shaoying Xu <shaoyi@...zon.com>
> Cc: stable@...r.kernel.org

Thanks for the patch. It seems to have fallen through the cracks. It looks
good just some nits: The timeout will be still dependent on the HZ value
because we use jiffie-granular timer.  But yes, I guess it is unnecessary
to make the imprecision 10x worse when we know we are likely dealing with
small numbers. 

> @@ -3460,14 +3460,13 @@ static int ext4_run_li_request(struct ext4_li_request *elr)
>  		ret = 1;
>  
>  	if (!ret) {

Please add a comment here so that we don't forget. Like:
		/* Use ns-granular time as init can be really fast */

With this feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

> -		timeout = jiffies;
> +		start_time = ktime_get_real_ns();
>  		ret = ext4_init_inode_table(sb, group,
>  					    elr->lr_timeout ? 0 : 1);
>  		trace_ext4_lazy_itable_init(sb, group);
>  		if (elr->lr_timeout == 0) {
> -			timeout = (jiffies - timeout) *
> -				EXT4_SB(elr->lr_super)->s_li_wait_mult;
> -			elr->lr_timeout = timeout;
> +			elr->lr_timeout = nsecs_to_jiffies((ktime_get_real_ns() - start_time) *
> +				EXT4_SB(elr->lr_super)->s_li_wait_mult);
>  		}
>  		elr->lr_next_sched = jiffies + elr->lr_timeout;
>  		elr->lr_next_group = group + 1;


								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR