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Date:   Thu, 12 Mar 2020 10:02:42 -0700
From:   Jaegeuk Kim <jaegeuk@kernel.org>
To:     Sahitya Tummala <stummala@codeaurora.org>
Cc:     Chao Yu <yuchao0@huawei.com>,
        linux-f2fs-devel@lists.sourceforge.net,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] f2fs: fix long latency due to discard during umount
Message-ID: <20200312170242.GA185506@google.com>
References: <1584011671-20939-1-git-send-email-stummala@codeaurora.org>
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On 03/12, Sahitya Tummala wrote:
> F2FS already has a default timeout of 5 secs for discards that
> can be issued during umount, but it can take more than the 5 sec
> timeout if the underlying UFS device queue is already full and there
> are no more available free tags to be used. In that case, submit_bio()
> will wait for the already queued discard requests to complete to get
> a free tag, which can potentially take way more than 5 sec.
> 
> Fix this by submitting the discard requests with REQ_NOWAIT
> flags during umount. This will return -EAGAIN for UFS queue/tag full
> scenario without waiting in the context of submit_bio(). The FS can
> then handle these requests by retrying again within the stipulated
> discard timeout period to avoid long latencies.
> 
> Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
> ---
>  fs/f2fs/segment.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> index fb3e531..a06bbac 100644
> --- a/fs/f2fs/segment.c
> +++ b/fs/f2fs/segment.c
> @@ -1124,10 +1124,13 @@ static int __submit_discard_cmd(struct f2fs_sb_info *sbi,
>  	struct discard_cmd_control *dcc = SM_I(sbi)->dcc_info;
>  	struct list_head *wait_list = (dpolicy->type == DPOLICY_FSTRIM) ?
>  					&(dcc->fstrim_list) : &(dcc->wait_list);
> -	int flag = dpolicy->sync ? REQ_SYNC : 0;
> +	int flag;
>  	block_t lstart, start, len, total_len;
>  	int err = 0;
>  
> +	flag = dpolicy->sync ? REQ_SYNC : 0;
> +	flag |= dpolicy->type == DPOLICY_UMOUNT ? REQ_NOWAIT : 0;
> +
>  	if (dc->state != D_PREP)
>  		return 0;
>  
> @@ -1203,6 +1206,11 @@ static int __submit_discard_cmd(struct f2fs_sb_info *sbi,
>  		bio->bi_end_io = f2fs_submit_discard_endio;
>  		bio->bi_opf |= flag;
>  		submit_bio(bio);
> +		if ((flag & REQ_NOWAIT) && (dc->error == -EAGAIN)) {
> +			dc->state = D_PREP;
> +			err = dc->error;
> +			break;
> +		}
>  
>  		atomic_inc(&dcc->issued_discard);
>  
> @@ -1510,6 +1518,10 @@ static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
>  			}
>  
>  			__submit_discard_cmd(sbi, dpolicy, dc, &issued);
> +			if (dc->error == -EAGAIN) {
> +				congestion_wait(BLK_RW_ASYNC, HZ/50);

						--> need to be DEFAULT_IO_TIMEOUT

> +				__relocate_discard_cmd(dcc, dc);

It seems we need to submit bio first, and then move dc to wait_list, if there's
no error, in __submit_discard_cmd().

> +			}
>  
>  			if (issued >= dpolicy->max_requests)
>  				break;
> -- 
> Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc.
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.