From a219ccf4637396a2392bfbec7c12acbfe2b06b46 Mon Sep 17 00:00:00 2001 From: "Srivatsa S. Bhat" Date: Fri, 6 Jun 2014 14:37:05 -0700 Subject: [PATCH] smp: print more useful debug info upon receiving IPI on an offline CPU There is a longstanding problem related to CPU hotplug which causes IPIs to be delivered to offline CPUs, and the smp-call-function IPI handler code prints out a warning whenever this is detected. Every once in a while this (usually harmless) warning gets reported on LKML, but so far it has not been completely fixed. Usually the solution involves finding out the IPI sender and fixing it by adding appropriate synchronization with CPU hotplug. However, while going through one such internal bug reports, I found that there is a significant bug in the receiver side itself (more specifically, in stop-machine) that can lead to this problem even when the sender code is perfectly fine. This patchset fixes that synchronization problem in the CPU hotplug stop-machine code. Patch 1 adds some additional debug code to the smp-call-function framework, to help debug such issues easily. Patch 2 modifies the stop-machine code to ensure that any IPIs that were sent while the target CPU was online, would be noticed and handled by that CPU without fail before it goes offline. Thus, this avoids scenarios where IPIs are received on offline CPUs (as long as the sender uses proper hotplug synchronization). In fact, I debugged the problem by using Patch 1, and found that the payload of the IPI was always the block layer's trigger_softirq() function. But I was not able to find anything wrong with the block layer code. That's when I started looking at the stop-machine code and realized that there is a race-window which makes the IPI _receiver_ the culprit, not the sender. Patch 2 fixes that race and hence this should put an end to most of the hard-to-debug IPI-to-offline-CPU issues. This patch (of 2): Today the smp-call-function code just prints a warning if we get an IPI on an offline CPU. This info is sufficient to let us know that something went wrong, but often it is very hard to debug exactly who sent the IPI and why, from this info alone. In most cases, we get the warning about the IPI to an offline CPU, immediately after the CPU going offline comes out of the stop-machine phase and reenables interrupts. Since all online CPUs participate in stop-machine, the information regarding the sender of the IPI is already lost by the time we exit the stop-machine loop. So even if we dump the stack on each CPU at this point, we won't find anything useful since all of them will show the stack-trace of the stopper thread. So we need a better way to figure out who sent the IPI and why. To achieve this, when we detect an IPI targeted to an offline CPU, loop through the call-single-data linked list and print out the payload (i.e., the name of the function which was supposed to be executed by the target CPU). This would give us an insight as to who might have sent the IPI and help us debug this further. [akpm@linux-foundation.org: correctly suppress warning output on second and later occurrences] Signed-off-by: Srivatsa S. Bhat Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: Ingo Molnar Cc: Tejun Heo Cc: Rusty Russell Cc: Frederic Weisbecker Cc: Christoph Hellwig Cc: Mel Gorman Cc: Rik van Riel Cc: Borislav Petkov Cc: Steven Rostedt Cc: Mike Galbraith Cc: Gautham R Shenoy Cc: "Paul E. McKenney" Cc: Oleg Nesterov Cc: Rafael J. Wysocki Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/smp.c | 18 +++++++++++++++--- 1 file changed, 15 insertions(+), 3 deletions(-) diff --git a/kernel/smp.c b/kernel/smp.c index 06d574e..306f818 100644 --- a/kernel/smp.c +++ b/kernel/smp.c @@ -185,14 +185,26 @@ void generic_smp_call_function_single_interrupt(void) { struct llist_node *entry; struct call_single_data *csd, *csd_next; + static bool warned; + + entry = llist_del_all(&__get_cpu_var(call_single_queue)); + entry = llist_reverse_order(entry); /* * Shouldn't receive this interrupt on a cpu that is not yet online. */ - WARN_ON_ONCE(!cpu_online(smp_processor_id())); + if (unlikely(!cpu_online(smp_processor_id()) && !warned)) { + warned = true; + WARN(1, "IPI on offline CPU %d\n", smp_processor_id()); - entry = llist_del_all(&__get_cpu_var(call_single_queue)); - entry = llist_reverse_order(entry); + /* + * We don't have to use the _safe() variant here + * because we are not invoking the IPI handlers yet. + */ + llist_for_each_entry(csd, entry, llist) + pr_warn("IPI callback %pS sent to offline CPU\n", + csd->func); + } llist_for_each_entry_safe(csd, csd_next, entry, llist) { csd->func(csd->info); -- 2.7.4