Commit b0473dcd authored by Paul E. McKenney's avatar Paul E. McKenney Committed by Thomas Gleixner
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smp: Improve smp_call_function_single() CSD-lock diagnostics 



Both smp_call_function() and smp_call_function_single() use per-CPU
call_single_data_t variable to hold the infamous CSD lock.  However,
while smp_call_function() acquires the destination CPU's CSD lock,
smp_call_function_single() instead uses the source CPU's CSD lock.
(These are two separate sets of CSD locks, cfd_data and csd_data,
respectively.)

This otherwise inexplicable pair of choices is explained by their
respective queueing properties.  If smp_call_function() where to
use the sending CPU's CSD lock, that would serialize the destination
CPUs' IPI handlers and result in long smp_call_function() latencies,
especially on systems with large numbers of CPUs.  For its part, if
smp_call_function_single() were to use the (single) destination CPU's
CSD lock, this would similarly serialize in the case where many CPUs
are sending IPIs to a single "victim" CPU.  Plus it would result in
higher levels of memory contention.

Except that if there is no NMI-based stack tracing on a weakly ordered
system where remote unsynchronized stack traces are especially unreliable,
the improved debugging beats the improved queueing.  This improved queueing
only matters if a bunch of CPUs are calling smp_call_function_single()
concurrently for a single "victim" CPU, which is not the common case.

Therefore, make smp_call_function_single() use the destination CPU's
csd_data instance in kernels built with CONFIG_CSD_LOCK_WAIT_DEBUG=y
where csdlock_debug_enabled is also set.  Otherwise, continue to use
the source CPU's csd_data.

Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
Signed-off-by: default avatarThomas Gleixner <tglx@kernel.org>
Reviewed-by: default avatarUlf Hansson <ulf.hansson@linaro.org>
Link: https://patch.msgid.link/25c2eb97-77c8-49a5-80ac-efe78dea272c@paulmck-laptop
parent ec39780d

kernel/smp.c

+15 −1
Original line number Diff line number Diff line
@@ -377,6 +377,20 @@ static __always_inline void csd_unlock(call_single_data_t *csd) 


static DEFINE_PER_CPU_SHARED_ALIGNED(call_single_data_t, csd_data);



#ifdef CONFIG_CSD_LOCK_WAIT_DEBUG

static call_single_data_t *get_single_csd_data(int cpu)

{

	if (static_branch_unlikely(&csdlock_debug_enabled))

		return per_cpu_ptr(&csd_data, cpu);

	return this_cpu_ptr(&csd_data);

}

#else

static call_single_data_t *get_single_csd_data(int cpu)

{

	return this_cpu_ptr(&csd_data);

}

#endif



void __smp_call_single_queue(int cpu, struct llist_node *node)

{

	/*
@@ -671,7 +685,7 @@ int smp_call_function_single(int cpu, smp_call_func_t func, void *info, 


	csd = &csd_stack;

	if (!wait) {

		csd = this_cpu_ptr(&csd_data);

		csd = get_single_csd_data(cpu);

		csd_lock(csd);

	}