On Thu, Nov 16 2006, Paul E. McKenney wrote:
> On Thu, Nov 16, 2006 at 10:06:25PM -0500, Alan Stern wrote:
> > On Thu, 16 Nov 2006, Linus Torvalds wrote:
> >
> > >
> > >
> > > On Thu, 16 Nov 2006, Alan Stern wrote:
> > > > On Thu, 16 Nov 2006, Linus Torvalds wrote:
> > > > >
> > > > > Paul, it would be _really_ nice to have some way to just initialize
> > > > > that SRCU thing statically. This kind of crud is just crazy.
> > > >
> > > > I looked into this back when SRCU was first added. It's essentially
> > > > impossible to do it, because the per-cpu memory allocation & usage APIs
> > > > are completely different for the static and the dynamic cases.
> > >
> > > I don't think that's how you'd want to do it.
> > >
> > > There's no way to do an initialization of a percpu allocation statically.
> > > That's pretty obvious.
> >
> > Hmmm... What about DEFINE_PER_CPU in include/asm-generic/percpu.h
> > combined with setup_per_cpu_areas() in init/main.c? So long as you want
> > all the CPUs to start with the same initial values, it should work.
> >
> > > What I'd suggest instead, is to make the allocation dynamic, and make it
> > > inside the srcu functions (kind of like I did now, but I did it at a
> > > higher level).
> > >
> > > Doing it at the high level was trivial right now, but we may well end up
> > > hitting this problem again if people start using SRCU more. Right now I
> > > suspect the cpufreq notifier is the only thing that uses SRCU, and it
> > > already showed this problem with SRCU initializers.
> > >
> > > So I was more thinking about moving my "one special case high level hack"
> > > down lower, down to the SRCU level, so that we'll never see _more_ of
> > > those horrible hacks. We'll still have the hacky thing, but at least it
> > > will be limited to a single place - the SRCU code itself.
> >
> > Another possible approach (but equally disgusting) is to use this static
> > allocation approach, and have the SRCU structure include both a static and
> > a dynamic percpu pointer together with a flag indicating which should be
> > used.
>
> I am actually taking some suggestions you made some months ago. At the
> time, I rejected them because they injected extra branches into the
> fastpath. However, recent experience indicates that you (Alan Stern)
> were right and I was wrong -- turns out that the update-side overhead
> cannot be so lightly disregarded, which forces memory barriers (but
> neither atomics nor cache misses) into the fastpath. If some application
> ends up being provably inconvenienced by the read-side overhead, they old
> implementation can be re-introduced under a different name or some such.
>
> So, here is my current plan:
>
> o Add NULL checks on srcu_struct_array to srcu_read_lock(),
> srcu_read_unlock(), and synchronize_srcu. These will
> acquire the mutex and attempt to initialize. If out
> of memory, they will use the new hardluckref field.
>
> o Add memory barriers to srcu_read_lock() and srcu_read_unlock().
>
> o Also add a memory barrier or two to synchronize_srcu(), which,
> in combination with those in srcu_read_lock() and srcu_read_unlock(),
> permit removing two of the three synchronize_sched() calls
> in synchronize_srcu(), decreasing its latency by roughly
> a factor of three.
>
> This change should have the added benefit of making
> synchronize_srcu() much easier to understand.
>
> o I left out the super-fastpath synchronize_srcu() because
> after sleeping on it, it scared me silly. Might be OK,
> but needs careful thought. The fastpath is of the form:
>
> if (srcu_readers_active(sp) == 0) {
> smp_mb();
> return;
> }
>
> prior to the mutex_lock() in synchronize_srcu().
It works for me, but the overhead is still large. Before it would take
8-12 jiffies for a synchronize_srcu() to complete without there actually
being any reader locks active, now it takes 2-3 jiffies. So it's
definitely faster, and as suspected the loss of two of three
synchronize_sched() cut down the overhead to a third.
It's still too heavy for me, by far the most calls I do to
synchronize_srcu() doesn't have any reader locks pending. I'm still a
big advocate of the fastpath srcu_readers_active() check. I can
understand the reluctance to make it the default, but for my case it's
"safe enough", so if we could either export srcu_readers_active() or
export a synchronize_srcu_fast() (or something like that), then SRCU
would be a good fit for barrier vs plug rework.
> Attached is a patch that compiles, but probably goes down in flames
> otherwise.
Works here :-)
--
Jens Axboe
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