Linus,
Do you have any objections in pulling this for 3.8? I added the entire
git diff below. Note, this has been in linux-next for a while too.
-- Steve
On Mon, 2012-12-17 at 15:42 +0100, Frederic Weisbecker wrote:
> Linus,
>
> We are currently working on extending the dynticks mode to broader contexts
> than just idle.
> Under some conditions on a busy CPU, the tick can be avoided (no need of
> preemption for one
> task running, no need of RCU state machine maintainance in userspace, etc...).
>
> The most popular application of this is the implementation of CPU isolation.
> On HPC
> workloads, where people run one task per-CPU in order to maximize the CPU
> performances,
> the kernel sets itself too much on the way with these often unnecessary
> interrupts.
>
> The result is a performance loss due to stolen CPU time and cache trashing of
> the userspace workset.
>
> Now CPU isolation is the most famous user. I expect more. For example we
> should be able
> to avoid the tick when we run in guest mode. And more generally this may be a
> win
> for most CPU-bound workloads.
>
> So in order to implement this full dynticks mode, we need to find
> alternatives to
> handle the many maintainance operations performed periodically and turn them
> to
> more one-shot event driven solutions.
>
> printk() is part of the problem. It must be safely callable from most places
> and for
> that purpose it performs an asynchronous wake up of the readers by probing on
> the tick for
> pending messages and readers through printk_tick().
>
> Of course if we use printk while the tick is stopped, the pending readers may
> not be woken
> up for a while. So a solution to make printk() working even if the CPU is in
> dynticks mode
> is to use the irq_work subsystem. This subsystem is typically able to fire
> self-IPIs.
> So when printk() is called, it now enqueues an irq_work that does the
> asynchronous wakeup:
>
> * If the tick is stopped, it raises a self-IPI
> * If the tick is running periodically then don't fire a self-IPI but wait for
> the next tick
> to handle that instead (irq work probes on the timer tick). This avoids
> self-IPIs storm in
> case of frequent printk() in short periods of time.
>
> I know this is a sensitive area. We want printk() to stay minimal and not
> rely too much
> on other subsystems that add complications and that may use printk themselves.
> That's why we chose irq_work because:
>
> - It's pretty small and self-contained
> - It's lockless
> - It handles most recursivity cases (if it uses printk() itself from the IPI
> path, this won't
> fire another IPI)
>
> But because it's sensitive, I'm proposing it as an RFC pull request.
>
> So if you're ok with that, please pull from:
>
> git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
> tags/printk-dynticks-for-linus
>
> HEAD: 74876a98a87a115254b3a66a14b27320b7f0acaa "printk: Wake up klogd using
> irq_work"
>
> It has been in linux-next.
>
> Thanks.
>
>
> Support for printk in dynticks mode:
>
> * Fix two races in irq work claiming
>
> * Generalize irq_work support to all archs
>
> * Don't stop tick with irq works pending. This
> fix is generally useful and concerns archs that
> can't raise self IPIs.
>
> * Flush irq works before CPU offlining.
>
> * Introduce "lazy" irq works that can wait for the
> next tick to be executed, unless it's stopped.
>
> * Implement klogd wake up using irq work. This
> removes the ad-hoc printk_tick()/printk_needs_cpu()
> hooks and make it working even in dynticks mode.
>
> Signed-off-by: Frederic Weisbecker
>
>
> Frederic Weisbecker (7):
> irq_work: Fix racy IRQ_WORK_BUSY flag setting
> irq_work: Fix racy check on work pending flag
> irq_work: Remove CONFIG_HAVE_IRQ_WORK
> nohz: Add API to check tick state
> irq_work: Don't stop the tick with pending works
> irq_work: Make self-IPIs optable
> printk: Wake up klogd using irq_work
>
> Steven Rostedt (2):
> irq_work: Flush work on CPU_DYING
> irq_work: Warn if there's still work on cpu_down
>
> arch/alpha/Kconfig |1 -
> arch/arm/Kconfig|1 -
> arch/arm64/Kconfig |1 -
> arch/blackfin/Kconfig |1 -
> arch/frv/Kconfig|1 -
> arch/hexagon/Kconfig|1 -
> arch/mips/Kconfig |1 -
> arch/parisc/Kconfig |1 -
> arch/powerpc/Kconfig|1 -
> arch/s390/Kconfig |1 -
> arch/sh/Kconfig |1 -
> arch/sparc/Kconfig |1 -
> arch/x86/Kconfig|1 -
> drivers/staging/iio/trigger/Kconfig |1 -
> include/linux/irq_work.h| 20 +
> include/linux/printk.h |3 -
>
