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Commit 71454272 authored by Milton Miller's avatar Milton Miller Committed by Benjamin Herrenschmidt
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powerpc: Use bytes instead of bitops in smp ipi multiplexing 



Since there are only 4 messages, we can replace the atomic bit set
(which uses atomic load reserve and store conditional sequence) with
a byte stores to seperate bytes.  We still have to perform a load
reserve and store conditional sequence to avoid loosing messages on
reception but we can do that with a single call to xchg.

The do {} while and __BIG_ENDIAN specific mask testing was chosen by
looking at the generated asm code.  On gcc-4.4, the bit masking becomes
a simple bit mask and test of the register returned from xchg without
storing and loading the value to the stack like attempts with a union
of bytes and an int (or worse, loading single bit constants from the
constant pool into non-voliatle registers that had to be preseved on
the stack).  The do {} while avoids an unconditional branch to the
end of the loop to test the entry / repeat condition of a while loop
and instead optimises for the expected single iteration of the loop.

We have a full mb() at the beginning to cover ordering between send,
ipi, and receive so we can use xchg_local and forgo the further
acquire and release barriers of xchg.

Signed-off-by: default avatarMilton Miller <miltonm@bga.com>
Signed-off-by: default avatarBenjamin Herrenschmidt <benh@kernel.crashing.org>
parent 1ece355b

arch/powerpc/kernel/smp.c

+18 −13
Original line number Diff line number Diff line
@@ -180,7 +180,7 @@ int smp_request_message_ipi(int virq, int msg) 


#ifdef CONFIG_PPC_SMP_MUXED_IPI

struct cpu_messages {

	unsigned long messages;		/* current messages bits */

	int messages;			/* current messages */

	unsigned long data;		/* data for cause ipi */

};

static DEFINE_PER_CPU_SHARED_ALIGNED(struct cpu_messages, ipi_message);
@@ -195,9 +195,9 @@ void smp_muxed_ipi_set_data(int cpu, unsigned long data) 
void smp_muxed_ipi_message_pass(int cpu, int msg)

{

	struct cpu_messages *info = &per_cpu(ipi_message, cpu);

	unsigned long *tgt = &info->messages;

	char *message = (char *)&info->messages;



	set_bit(msg, tgt);

	message[msg] = 1;

	mb();

	smp_ops->cause_ipi(cpu, info->data);

}
@@ -205,30 +205,35 @@ void smp_muxed_ipi_message_pass(int cpu, int msg) 
void smp_muxed_ipi_resend(void)

{

	struct cpu_messages *info = &__get_cpu_var(ipi_message);

	unsigned long *tgt = &info->messages;



	if (*tgt)

	if (info->messages)

		smp_ops->cause_ipi(smp_processor_id(), info->data);

}



irqreturn_t smp_ipi_demux(void)

{

	struct cpu_messages *info = &__get_cpu_var(ipi_message);

	unsigned long *tgt = &info->messages;

	unsigned int all;



	mb();	/* order any irq clear */

	while (*tgt) {

		if (test_and_clear_bit(PPC_MSG_CALL_FUNCTION, tgt))



	do {

		all = xchg_local(&info->messages, 0);



#ifdef __BIG_ENDIAN

		if (all & (1 << (24 - 8 * PPC_MSG_CALL_FUNCTION)))

			generic_smp_call_function_interrupt();

		if (test_and_clear_bit(PPC_MSG_RESCHEDULE, tgt))

		if (all & (1 << (24 - 8 * PPC_MSG_RESCHEDULE)))

			reschedule_action(0, NULL); /* upcoming sched hook */

		if (test_and_clear_bit(PPC_MSG_CALL_FUNC_SINGLE, tgt))

		if (all & (1 << (24 - 8 * PPC_MSG_CALL_FUNC_SINGLE)))

			generic_smp_call_function_single_interrupt();

#if defined(CONFIG_DEBUGGER) || defined(CONFIG_KEXEC)

		if (test_and_clear_bit(PPC_MSG_DEBUGGER_BREAK, tgt))

		if (all & (1 << (24 - 8 * PPC_MSG_DEBUGGER_BREAK)))

			debug_ipi_action(0, NULL);

#else

#error Unsupported ENDIAN

#endif

	}

	} while (info->messages);



	return IRQ_HANDLED;

}

#endif /* CONFIG_PPC_SMP_MUXED_IPI */