Re: [PATCH] ext4: add cond_resched() to __ext4_find_entry()

From: Jan Kara <jack@suse.cz>
To: Shijie Luo <luoshijie1@huawei.com>
Cc: linux-ext4@vger.kernel.org, tytso@mit.edu, jack@suse.cz,
	lutianxiong@huawei.com
Subject: Re: [PATCH] ext4: add cond_resched() to __ext4_find_entry()
Date: Mon, 17 Feb 2020 11:31:35 +0100	[thread overview]
Message-ID: <20200217103135.GD12032@quack2.suse.cz> (raw)
In-Reply-To: <20200215080206.13293-1-luoshijie1@huawei.com>

On Sat 15-02-20 03:02:06, Shijie Luo wrote:
> We tested a soft lockup problem in linux 4.19 which could also
> be found in linux 5.x.
> 
> When dir inode takes up a large number of blocks, and if the
> directory is growing when we are searching, it's possible the
> restart branch could be called many times, and the do while loop
> could hold cpu a long time.
> 
> Here is the call trace in linux 4.19.
> 
> [  473.756186] Call trace:
> [  473.756196]  dump_backtrace+0x0/0x198
> [  473.756199]  show_stack+0x24/0x30
> [  473.756205]  dump_stack+0xa4/0xcc
> [  473.756210]  watchdog_timer_fn+0x300/0x3e8
> [  473.756215]  __hrtimer_run_queues+0x114/0x358
> [  473.756217]  hrtimer_interrupt+0x104/0x2d8
> [  473.756222]  arch_timer_handler_virt+0x38/0x58
> [  473.756226]  handle_percpu_devid_irq+0x90/0x248
> [  473.756231]  generic_handle_irq+0x34/0x50
> [  473.756234]  __handle_domain_irq+0x68/0xc0
> [  473.756236]  gic_handle_irq+0x6c/0x150
> [  473.756238]  el1_irq+0xb8/0x140
> [  473.756286]  ext4_es_lookup_extent+0xdc/0x258 [ext4]
> [  473.756310]  ext4_map_blocks+0x64/0x5c0 [ext4]
> [  473.756333]  ext4_getblk+0x6c/0x1d0 [ext4]
> [  473.756356]  ext4_bread_batch+0x7c/0x1f8 [ext4]
> [  473.756379]  ext4_find_entry+0x124/0x3f8 [ext4]
> [  473.756402]  ext4_lookup+0x8c/0x258 [ext4]
> [  473.756407]  __lookup_hash+0x8c/0xe8
> [  473.756411]  filename_create+0xa0/0x170
> [  473.756413]  do_mkdirat+0x6c/0x140
> [  473.756415]  __arm64_sys_mkdirat+0x28/0x38
> [  473.756419]  el0_svc_common+0x78/0x130
> [  473.756421]  el0_svc_handler+0x38/0x78
> [  473.756423]  el0_svc+0x8/0xc
> [  485.755156] watchdog: BUG: soft lockup - CPU#2 stuck for 22s! [tmp:5149]
> 
> Add cond_resched() to avoid soft lockup and to provide a better
> system responding.
> 
> Signed-off-by: Shijie Luo <luoshijie1@huawei.com>

Thanks for the patch! It looks good. You can add:

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

I was just wondering how come we didn't hit other scheduling points (e.g.
when reading directory blocks from the disk) but likely the directory
including new blocks was staying in cache...

								Honza

> ---
>  fs/ext4/namei.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
> index 129d2ebae00d..9a1836632f51 100644
> --- a/fs/ext4/namei.c
> +++ b/fs/ext4/namei.c
> @@ -1511,6 +1511,7 @@ static struct buffer_head *__ext4_find_entry(struct inode *dir,
>  		/*
>  		 * We deal with the read-ahead logic here.
>  		 */
> +		cond_resched();
>  		if (ra_ptr >= ra_max) {
>  			/* Refill the readahead buffer */
>  			ra_ptr = 0;
> -- 
> 2.19.1
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR