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    SubjectRe: [PATCH -v2] use per cpu data for single cpu ipi calls

    * Peter Zijlstra <peterz@infradead.org> wrote:

    > On Thu, 2009-01-29 at 10:08 -0500, Steven Rostedt wrote:
    > > The smp_call_function can be passed a wait parameter telling it to
    > > wait for all the functions running on other CPUs to complete before
    > > returning, or to return without waiting. Unfortunately, this is
    > > currently just a suggestion and not manditory. That is, the
    > > smp_call_function can decide not to return and wait instead.
    > >
    > > The reason for this is because it uses kmalloc to allocate storage
    > > to send to the called CPU and that CPU will free it when it is done.
    > > But if we fail to allocate the storage, the stack is used instead.
    > > This means we must wait for the called CPU to finish before
    > > continuing.
    > >
    > > Unfortunatly, some callers do no abide by this hint and act as if
    > > the non-wait option is mandatory. The MTRR code for instance will
    > > deadlock if the smp_call_function is set to wait. This is because
    > > the smp_call_function will wait for the other CPUs to finish their
    > > called functions, but those functions are waiting on the caller to
    > > continue.
    > >
    > > This patch changes the generic smp_call_function code to use per cpu
    > > variables if the allocation of the data fails for a single CPU call. The
    > > smp_call_function_many will fall back to the smp_call_function_single
    > > if it fails its alloc. The smp_call_function_single is modified
    > > to not force the wait state.
    > >
    > > Since we now are using a single data per cpu we must synchronize the
    > > callers to prevent a second caller modifying the data before the
    > > first called IPI functions complete. To do so, I added a flag to
    > > the call_single_data called CSD_FLAG_LOCK. When the single CPU is
    > > called (which can be called when a many call fails an alloc), we
    > > set the LOCK bit on this per cpu data. When the caller finishes
    > > it clears the LOCK bit.
    > >
    > > The caller must wait till the LOCK bit is cleared before setting
    > > it. When it is cleared, there is no IPI function using it.
    > > A spinlock is used to synchronize the setting of the bit between
    > > callers. Since only one callee can be called at a time, and it
    > > is the only thing to clear it, the IPI does not need to use
    > > any locking.
    > >
    > > [
    > > changes for v2:
    > >
    > > -- kept kmalloc and only use per cpu if kmalloc fails.
    > > (Requested by Peter Zijlstra)
    > >
    > > -- added per cpu spinlocks
    > > (Requested by Andrew Morton and Peter Zijlstra)
    > > ]
    > >
    > > Signed-off-by: Steven Rostedt <srostedt@redhat.com>
    >
    > Looks nice, thanks!
    >
    > Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

    started testing it in tip/core/urgent, thanks guys!

    Ingo


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