On Thu, 12 Sep 2013, Mathieu Desnoyers wrote:
> * Peter Zijlstra (pet...@infradead.org) wrote:
> [...]
> > Yep, that's good. I suppose if there's multiple use sites we can jump
> > through another few hoops to get rid of the specific struct foo
> > assumptions by storing sizeof() whatever we do use and playing pointer
> > math games.
> >
> > But for now with the time stuff as only user this looks ok.
>
> OK! Here is the full implementation of the idea against Linux
> timekeeper, ntp, and PPS. It appears that ntp and PPS were relying on
> the timekeeper seqlock too. And guess what, after booting my laptop with
> this kernel there still no smoke coming out of it after a good 5 minutes
> of testing. ;-)
>
> Comments are welcome.
First of all this needs to be split into several patches.
> static void update_wall_time(void)
...
> - write_seqcount_begin(&timekeeper_seq);
> /* Update clock->cycle_last with the new value */
> clock->cycle_last = tk->cycle_last;
> - /*
> - * Update the real timekeeper.
> - *
> - * We could avoid this memcpy by switching pointers, but that
> - * requires changes to all other timekeeper usage sites as
> - * well, i.e. move the timekeeper pointer getter into the
> - * spinlocked/seqcount protected sections. And we trade this
> - * memcpy under the timekeeper_seq against one before we start
> - * updating.
> - */
> - memcpy(real_tk, tk, sizeof(*tk));
> - timekeeping_update(real_tk, action);
So you're dropping the timekeeping_update() call here. I wonder how
this works.
> + timekeeper_write_begin(&latch_timekeeper, &prev, &tk);
> + *tk = *prev;
Do we really need to do the copy on all call sites, instead of doing
it in timekeeper_write_begin() ?
Thanks,
tglx
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