Re: PROBLEM: CONFIG_NO_HZ could cause software timeouts

[Corrado Zoccolo <czoccolo@gmail.com>]

Is XIP enabled? It seems that the xip_udelay is not very accurate, and
may be the cause of what you are seeing.

On Thu, Sep 3, 2009 at 12:50 PM, Norbert van
Bolhuis<nvbolhuis@aimvalley.nl> wrote:
>
> The problem occurs when e.g. drivers use time_after(jiffes, timeout).
>
> CONFIG_NO_HZ could make jiffies advance by more than 1.
> This is done by:
> tick_nohz_update_jiffies->tick_do_update_jiffies64->do_timer
>
> If drivers use a timeout value of jiffies+1,
> "time_after(jiffies, timeout)" will be true after 1 interrupt
> (given that it advances jiffies by at least 2).
>
> This is exactly what happens in cfi_cmdset_0002.c:do_write_buffer
> for our case (Powerpc MPC8313, linux-2.6.28, CONFIG_HZ=250, CONFIG_NO_HZ=y).
>
> do_write_buffer does the following:
>  unsigned long uWriteTimeout = ( HZ / 1000 ) + 1;
>  ...
>  timeo = jiffies + uWriteTimeout;
>  ...
>  for (;;) {
>  ...
>  if (time_after(jiffies, timeo) && !chip_ready(map, adr))
>   break;
>  if (chip_ready(map, adr)) {
>   xip_enable(map, chip, adr);
>   goto op_done;
>  }
>  UDELAY(map, chip, adr, 1);
>  }
>  /* software timeout */
>  ret = -EIO;
> opdone:
>  ...
>
> I've seen a few software timeouts after the for-loop
> looped only 13 times (= 13 us delay, i.s.o. the expected 1 ms). Typically
> our NOR flash (S29GL01GP) may need upto ~ 200 us to be ready.
>
> disabling CONFIG_NO_HZ fixes the problem.
> replacing time_after by a for-loop counter to loop max 1000 times
> also fixes the problem.
>
> the latest kernel seems to have the same problem.
>
> do I miss something here or is this a known problem of CONFIG_NO_HZ ?
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-- 
__________________________________________________________________________

dott. Corrado Zoccolo                          mailto:czoccolo@gmail.com
PhD - Department of Computer Science - University of Pisa, Italy
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