Re: [PATCH] sky2: Use deferrable timer for watchdog

Arjan van de Ven wrote:

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My interpretation of the api is:
   * round_jiffies()  - timer wants to wakeup but isn't precise
about when so schedule
                        on next second when system will wake up anyway;
                        e.g why meetings are usually scheduled on
the hour

   * deferrable       - timer doesn't have to really wakeup but
wants to happen near
                        a particular time. e.g. "I'll meet you at
the pub around 8pm"

this is not correct.

deferrable means "if you're busy wake me up at this time. But if not,
don't bother waking up for me, get to it
later".

The "later" can be a LONG time later, several seconds easily, if not more.
(timers are on a per cpu bases, and you may end up with a several-core
system where the common timers are all on another cpu
than this one)

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If this is the case then the whole usage of round_jiffies() is bogus.
All users of round_jiffies()
should just be converted to deferrable??  I am a bit concerned that
if deferrable gets used everywhere
then a strange situation would occur where all timers were waiting
for some other timer to finally
happen, kind of a wierd timelock situation. Like the old chip/dale
cartoon:
 "you first, no you first, after you mister chip, no after you mister
dale,..."

that's a dangerous situation indeed and I'd really like to know what
the limits
are for deferring deferrable timers.... Arjan, do you know? Anyone?

there is NO limit to deferring a timer. Do NOT use a deferrable timer if
you can't afford the timer to not happen
within.. 10 to 100 seconds! (or more)
They are really meant for things where you CAN afford for it to not
happen when you're idle....

ok, that's just bad and if there's no user-defineable limit to the deferral I
definately don't like this change.

Can I safely assume that any irq will cause all deferred timers to run?

If this is the case then for e1000 this patch is still OK since the watchdog needs
to run (1) after a link up/down interrupt or (2) to update statistics. Those
statistics won't increase if there is no traffic of course...

Auke