linux-ext4 - Re: [REPORT] syscall reboot + umh + firmware fallback

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Date:   Mon, 23 May 2022 10:10:45 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Theodore Ts'o <tytso@....edu>
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Subject: Re: [REPORT] syscall reboot + umh + firmware fallback

On Thu, May 12, 2022 at 09:56:46AM -0400, Theodore Ts'o wrote:
> On Thu, May 12, 2022 at 08:18:24PM +0900, Byungchul Park wrote:
> > I have a question about this one. Yes, it would never been stuck thanks
> > to timeout. However, IIUC, timeouts are not supposed to expire in normal
> > cases. So I thought a timeout expiration means not a normal case so need
> > to inform it in terms of dependency so as to prevent further expiraton.
> > That's why I have been trying to track even timeout'ed APIs.
> 
> As I beleive I've already pointed out to you previously in ext4 and
> ocfs2, the jbd2 timeout every five seconds happens **all** the time
> while the file system is mounted.  Commits more frequently than five
> seconds is the exception case, at least for desktops/laptop workloads.

Thanks, Ted. It's easy to stop tracking APIs with timeout. I've been
just afraid that the cases that we want to suppress anyway will be
skipped.

However, I should stop it if it produces too many false alarms.

> We *don't* get to the timeout only when a userspace process calls
> fsync(2), or if the journal was incorrectly sized by the system
> administrator so that it's too small, and the workload has so many
> file system mutations that we have to prematurely close the
> transaction ahead of the 5 second timeout.

Yeah... It's how journaling works. Thanks.

> > Do you think DEPT shouldn't track timeout APIs? If I was wrong, I
> > shouldn't track the timeout APIs any more.
> 
> DEPT tracking timeouts will cause false positives in at least some
> cases.  At the very least, there needs to be an easy way to suppress
> these false positives on a per wait/mutex/spinlock basis.

The easy way is to stop tracking those that are along with timeout until
DEPT starts to consider waits/events by timeout functionality itself.

Thanks.

	Byungchul
> 
>       	       	    	     	      	   	 - Ted