Date:	Thu, 06 Jun 2013 01:39:06 +0300
From:	Imre Deak <imre.deak@...el.com>
To:	Brian Norris <computersforpeace@...il.com>
Cc:	Huang Shijie <b32955@...escale.com>,
	Artem Bityutskiy <dedekind1@...il.com>,
	linux-mtd@...ts.infradead.org,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Kevin Cernekee <cernekee@...il.com>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 3/3] mtd: cfi_cmdset_0002: increase do_write_buffer()
 timeout

On Wed, 2013-06-05 at 14:08 -0700, Brian Norris wrote:
> Adding a few others
> 
> For reference, this thread started with this patch:
> 
> http://lists.infradead.org/pipermail/linux-mtd/2013-June/047164.html
> 
> On Wed, Jun 5, 2013 at 11:01 AM, Brian Norris
> <computersforpeace@...il.com> wrote:
> > On Tue, Jun 4, 2013 at 12:03 AM, Huang Shijie <b32955@...escale.com> wrote:
> >> 于 2013年06月04日 09:46, Brian Norris 写道:
> >>> After various tests, it seems simply that the timeout is not long enough
> >>> for my system; increasing it by a few jiffies prevented all failures
> >>> (testing for 12+ hours). There is no harm in increasing the timeout, but
> >>> there is harm in having it too short, as evidenced here.
> >>>
> >> I like the patch1 and patch 2.
> >>
> >> But extending the timeout from 1ms to 10ms is like a workaround. :)
> >
> > I was afraid you might say that; that's why I stuck the first two
> > patches first ;)
> ...
> >> I GUESS your problem is caused by the timer system, not the MTD code. I
> >> ever met this type of bug.
> ...
> >> I try to describe the jiffies bug with my poor english:
> >>
> >> [1] background:
> >> CONFIG_HZ=100, CONFIG_NO_HZ=y
> >>
> >> [2] call nand_wait() when we write a nand page.
> >>
> >> [3] The jiffies was not updated at a _even_ speed.
> >>
> >> In the nand_wait(), you wait for 20ms(2 jiffies) for a page write,
> >> and the timeout occurs during the page write. Of course, you think that
> >> we have already waited for 20ms.
> >> But in actually, we only waited for 1ms or less!
> >> How do i know this? I use the gettimeofday to check the real time when
> >> the timeout occur.
> >
> > I suspected this very type of thing, since this has come up in a few
> > different contexts. And for some time, with a number of different
> > checks, it appeared that this *wasn't* the case. But while writing
> > this very email, I had the bright idea that my time checkpoint was in
> > slightly the wrong place; so sure enough, I found that I was timing
> > out after only 72519 ns! (That is, 72 us, or well below the max write
> > buffer time.)
> 
> So I can confirm that with the 1ms timeout, I actually am sometimes
> timing out at 40 to 70 microseconds. I think this may have multiple
> causes:
> (1) uneven timer interrupts, as suggested by Huang?
> (2) a jiffies timeout of 1 is two short (with HZ=1000, msecs_to_jiffies(1) is 1)
> 
> Regarding reason (2):
> 
> My thought (which matches with Imre's comments from his [1]) is that
> one problem here is that we do not know how long it will be until the
> *next* timer tick -- "waiting 1 jiffy" is really just waiting until
> the next timer tick, which very well might be in 40us! So the correct
> timeout calculation is something like:
> 
> uWriteTimeout = msecs_to_jiffies(1) + 1;
> 
> or with Imre's proposed methods (not merged upstream yet), just:
> 
> uWriteTimeout = msecs_to_jiffies_timeout(1);
> 
> Thoughts?

I think what you describe at (2) wouldn't cause a premature timeout in
your case. The driver uses the returned jiffy value something like the
following in all cases (before applying the patch with the +1 change):

uWriteTimeout = msecs_to_jiffies(1);
timeout = jiffies + uWriteTimeout;
while (!condition)
	if (time_after(jiffies, timeout))
		return -ETIMEDOUT;

Here using time_after() as opposed to time_after_eq() serves as an
implicit +1 and thus guarantees that you wait at least 1 msec.

A bit off-topic:
Though using msecs_to_jiffies() is not a problem here, I think in this
case and almost always it would need less thinking and thus be safer to
still use msecs_to_jiffies_timeout(). A rare exception would be when the
+1 adjustment would accumulate to a significant error, like in the
following polling loop:

for (i = 0; i <= 50; i++) {
	if (poll_condition)
		return 0;
	schedule_timeout(msecs_to_jiffies(1));
}
return -ETIMEDOUT;

Here on HZ=1000 we would time out in average after 100 msec using
msecs_to_jiffies_timeout(1), whereas the intention was 50 msecs. 

--Imre

> Note that a 2-jiffy timeout does not, in fact, totally resolve my
> problems; with a timeout of 2 jiffies, I still get a timeout that
> (according to getnstimeofday()) occurs after only 56us. It does
> decrease its rate of occurrence, but Huang may still be right that
> reason (1) is involved.
> 
> Brian
> 
> [1] http://marc.info/?l=linux-kernel&m=136854294730957


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