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    SubjectRe: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync
    On Fri, Nov 17, 2006 at 10:29:25AM +0100, Jens Axboe wrote:
    > 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.

    Good to hear, thank you for trying it out!

    > 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.

    OK, will export the interface. Do your queues have associated locking?

    > > Attached is a patch that compiles, but probably goes down in flames
    > > otherwise.
    >
    > Works here :-)

    I have at least a couple bugs that would show up under low-memory
    situations, will fix and post an update.

    Thanx, Paul
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