On Sun, Nov 11, 2012 at 04:45:09PM +0100, Oleg Nesterov wrote:
> On 11/09, Paul E. McKenney wrote:
> >
> > On Fri, Nov 09, 2012 at 07:10:48PM +0100, Oleg Nesterov wrote:
> > >
> > > static bool xxx(brw)
> > > {
> > > down_write(&brw->rw_sem);
> >
> > down_write_trylock()
> >
> > As you noted in your later email. Presumably you return false if
> > the attempt to acquire it fails.
>
> Yes, yes, thanks.
>
> > > But first we should do other changes, I think. IMHO we should not do
> > > synchronize_sched() under mutex_lock() and this will add (a bit) more
> > > complications. We will see.
> >
> > Indeed, that does put considerable delay on the writers. There is always
> > synchronize_sched_expedited(), I suppose.
>
> I am not sure about synchronize_sched_expedited() (at least unconditionally),
> but: only the 1st down_write() needs synchronize_, and up_write() do not
> need to sleep in synchronize_ at all.
>
> To simplify, lets ignore the fact that the writers need to serialize with
> each other. IOW, the pseudo-code below is obviously deadly wrong and racy,
> just to illustrate the idea.
>
> 1. We remove brw->writer_mutex and add "atomic_t writers_ctr".
>
> update_fast_ctr() uses atomic_read(brw->writers_ctr) == 0 instead
> of !mutex_is_locked().
>
> 2. down_write() does
>
> if (atomic_add_return(brw->writers_ctr) == 1) {
> // first writer
> synchronize_sched();
> ...
> } else {
> ... XXX: wait for percpu_up_write() from the first writer ...
> }
>
> 3. up_write() does
>
> if (atomic_dec_unless_one(brw->writers_ctr)) {
> ... wake up XXX writers above ...
> return;
> } else {
> // the last writer
> call_rcu_sched( func => { atomic_dec(brw->writers_ctr) } );
> }
Agreed, an asynchronous callback can be used to switch the readers
back onto the fastpath. Of course, as you say, getting it all working
will require some care. ;-)
> Once again, this all is racy, but hopefully the idea is clear:
>
> - down_write(brw) sleeps in synchronize_sched() only if brw
> has already switched back to fast-path-mode
>
> - up_write() never sleeps in synchronize_sched(), it uses
> call_rcu_sched() or wakes up the next writer.
>
> Of course I am not sure this all worth the trouble, this should be discussed.
> (and, cough, I'd like to add the multi-writers mode which I'm afraid nobody
> will like) But I am not going to even try to do this until the current patch
> is applied, I need it to fix the bug in uprobes and I think the current code
> is "good enough". These changes can't help to speedup the readers, and the
> writers are slow/rare anyway.
Probably best to wait for multi-writers until there is a measurable need,
to be sure! ;-)
Thanx, Paul
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