Messages in this thread | | | Date | Thu, 29 Jan 2009 17:17:12 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH -v2] use per cpu data for single cpu ipi calls |
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* 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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