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Commit d8e8c7dd authored by Vineet Gupta's avatar Vineet Gupta
Browse files

ARC: [SMP] optimize IPI send and receive 

* Don't send an IPI if receiver already has a pending IPI.
  Atomically piggyback the new msg with pending msg.

* IPI receiver looping on xchg() not required

References: https://lkml.org/lkml/2013/11/25/232


Suggested-by: default avatarPeter Zijlstra <peterz@infradead.org>
Signed-off-by: default avatarVineet Gupta <vgupta@synopsys.com>
parent f2a4aa56

arch/arc/kernel/smp.c

+40 −28
Original line number Diff line number Diff line
@@ -215,16 +215,31 @@ static DEFINE_PER_CPU(unsigned long, ipi_data); 
static void ipi_send_msg_one(int cpu, enum ipi_msg_type msg)

{

	unsigned long __percpu *ipi_data_ptr = per_cpu_ptr(&ipi_data, cpu);

	unsigned long old, new;

	unsigned long flags;



	pr_debug("%d Sending msg [%d] to %d\n", smp_processor_id(), msg, cpu);



	local_irq_save(flags);



	set_bit(msg, ipi_data_ptr);

	/*

	 * Atomically write new msg bit (in case others are writing too),

	 * and read back old value

	 */

	do {

		new = old = *ipi_data_ptr;

		new |= 1U << msg;

	} while (cmpxchg(ipi_data_ptr, old, new) != old);



	/* Call the platform specific cross-CPU call function  */

	if (plat_smp_ops.ipi_send)

	/*

	 * Call the platform specific IPI kick function, but avoid if possible:

	 * Only do so if there's no pending msg from other concurrent sender(s).

	 * Otherwise, recevier will see this msg as well when it takes the

	 * IPI corresponding to that msg. This is true, even if it is already in

	 * IPI handler, because !@old means it has not yet dequeued the msg(s)

	 * so @new msg can be a free-loader

	 */

	if (plat_smp_ops.ipi_send && !old)

		plat_smp_ops.ipi_send(cpu);



	local_irq_restore(flags);
@@ -269,12 +284,8 @@ static void ipi_cpu_stop(void) 
	machine_halt();

}



static inline void __do_IPI(unsigned long pending)

static inline void __do_IPI(unsigned long msg)

{

	while (pending) {



		unsigned long msg = __ffs(pending);



	switch (msg) {

	case IPI_RESCHEDULE:

		scheduler_ipi();
@@ -289,11 +300,7 @@ static inline void __do_IPI(unsigned long pending) 
		break;



	default:

			pr_warn("IPI missing msg\n");



		}



		pending &= ~(1U << msg);

		pr_warn("IPI with unexpected msg %ld\n", msg);

	}

}
@@ -312,11 +319,16 @@ irqreturn_t do_IPI(int irq, void *dev_id) 
		plat_smp_ops.ipi_clear(irq);



	/*

	 * XXX: is this loop really needed

	 * And do we need to move ipi_clean inside

	 * "dequeue" the msg corresponding to this IPI (and possibly other

	 * piggybacked msg from elided IPIs: see ipi_send_msg_one() above)

	 */

	while ((pending = xchg(this_cpu_ptr(&ipi_data), 0)) != 0)

		__do_IPI(pending);

	pending = xchg(this_cpu_ptr(&ipi_data), 0);



	do {

		unsigned long msg = __ffs(pending);

		__do_IPI(msg);

		pending &= ~(1U << msg);

	} while (pending);



	return IRQ_HANDLED;

}