From: "Jürgen Groß" <jgross@suse.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
paulmck@kernel.org, mhocko@suse.com,
Jonathan Corbet <corbet@lwn.net>
Subject: Re: [PATCH v2 3/3] kernel/smp: add more data to CSD lock debugging
Date: Mon, 1 Mar 2021 20:16:12 +0100 [thread overview]
Message-ID: <c7c1eeb5-21b3-339b-4b25-a6c8df820146@suse.com> (raw)
In-Reply-To: <YD0fTLnQTQ7/M7fx@hirez.programming.kicks-ass.net>
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On 01.03.21 18:07, Peter Zijlstra wrote:
> On Mon, Mar 01, 2021 at 11:13:36AM +0100, Juergen Gross wrote:
>> In order to help identifying problems with IPI handling and remote
>> function execution add some more data to IPI debugging code.
>>
>> There have been multiple reports of cpus looping long times (many
>> seconds) in smp_call_function_many() waiting for another cpu executing
>> a function like tlb flushing. Most of these reports have been for
>> cases where the kernel was running as a guest on top of KVM or Xen
>> (there are rumours of that happening under VMWare, too, and even on
>> bare metal).
>>
>> Finding the root cause hasn't been successful yet, even after more than
>> 2 years of chasing this bug by different developers.
>>
>> Commit 35feb60474bf4f7 ("kernel/smp: Provide CSD lock timeout
>> diagnostics") tried to address this by adding some debug code and by
>> issuing another IPI when a hang was detected. This helped mitigating
>> the problem (the repeated IPI unlocks the hang), but the root cause is
>> still unknown.
>>
>> Current available data suggests that either an IPI wasn't sent when it
>> should have been, or that the IPI didn't result in the target cpu
>> executing the queued function (due to the IPI not reaching the cpu,
>> the IPI handler not being called, or the handler not seeing the queued
>> request).
>>
>> Try to add more diagnostic data by introducing a global atomic counter
>> which is being incremented when doing critical operations (before and
>> after queueing a new request, when sending an IPI, and when dequeueing
>> a request). The counter value is stored in percpu variables which can
>> be printed out when a hang is detected.
>>
>> The data of the last event (consisting of sequence counter, source
>> cpu, target cpu, and event type) is stored in a global variable. When
>> a new event is to be traced, the data of the last event is stored in
>> the event related percpu location and the global data is updated with
>> the new event's data. This allows to track two events in one data
>> location: one by the value of the event data (the event before the
>> current one), and one by the location itself (the current event).
>>
>> A typical printout with a detected hang will look like this:
>>
>> csd: Detected non-responsive CSD lock (#1) on CPU#1, waiting 5000000003 ns for CPU#06 scf_handler_1+0x0/0x50(0xffffa2a881bb1410).
>> csd: CSD lock (#1) handling prior scf_handler_1+0x0/0x50(0xffffa2a8813823c0) request.
>> csd: cnt(00008cc): ffff->0000 dequeue (src cpu 0 == empty)
>> csd: cnt(00008cd): ffff->0006 idle
>> csd: cnt(0003668): 0001->0006 queue
>> csd: cnt(0003669): 0001->0006 ipi
>> csd: cnt(0003e0f): 0007->000a queue
>> csd: cnt(0003e10): 0001->ffff ping
>> csd: cnt(0003e71): 0003->0000 ping
>> csd: cnt(0003e72): ffff->0006 gotipi
>> csd: cnt(0003e73): ffff->0006 handle
>> csd: cnt(0003e74): ffff->0006 dequeue (src cpu 0 == empty)
>> csd: cnt(0003e7f): 0004->0006 ping
>> csd: cnt(0003e80): 0001->ffff pinged
>> csd: cnt(0003eb2): 0005->0001 noipi
>> csd: cnt(0003eb3): 0001->0006 queue
>> csd: cnt(0003eb4): 0001->0006 noipi
>> csd: cnt now: 0003f00
>>
>> The idea is to print only relevant entries. Those are all events which
>> are associated with the hang (so sender side events for the source cpu
>> of the hanging request, and receiver side events for the target cpu),
>> and the related events just before those (for adding data needed to
>> identify a possible race). Printing all available data would be
>> possible, but this would add large amounts of data printed on larger
>> configurations.
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> Tested-by: Paul E. McKenney <paulmck@kernel.org>
>> ---
>> V2:
>> - add automatic data deciphering and sorting of entries
>> - add new trace point for leaving flush_smp_call_function_queue()
>> - add information when finding an empty call_single_queue
>
> They do not apply on top of these:
>
> https://lkml.kernel.org/r/20210220231712.2475218-2-namit@vmware.com
They are already in tip locking/core (Ingo applied them).
Juergen
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next prev parent reply other threads:[~2021-03-02 2:29 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-01 10:13 [PATCH v2 0/3] kernel/smp.c: add more CSD lock debugging Juergen Gross
2021-03-01 10:13 ` [PATCH v2 1/3] kernel/smp: add boot parameter for controlling " Juergen Gross
2021-03-01 14:09 ` [tip: locking/core] locking/csd_lock: Add " tip-bot2 for Juergen Gross
2021-03-06 11:54 ` tip-bot2 for Juergen Gross
2021-03-01 10:13 ` [PATCH v2 2/3] kernel/smp: prepare more " Juergen Gross
2021-03-01 14:09 ` [tip: locking/core] locking/csd_lock: Prepare " tip-bot2 for Juergen Gross
2021-03-06 11:54 ` tip-bot2 for Juergen Gross
2021-03-01 10:13 ` [PATCH v2 3/3] kernel/smp: add more data to " Juergen Gross
2021-03-01 14:09 ` [tip: locking/core] locking/csd_lock: Add " tip-bot2 for Juergen Gross
2021-03-01 17:07 ` [PATCH v2 3/3] kernel/smp: add " Peter Zijlstra
2021-03-01 19:16 ` Jürgen Groß [this message]
2021-03-01 20:44 ` Peter Zijlstra
2021-03-02 7:05 ` Ingo Molnar
2021-03-02 7:14 ` Ingo Molnar
2021-03-02 5:57 ` Jürgen Groß
2021-03-06 11:54 ` [tip: locking/core] locking/csd_lock: Add " tip-bot2 for Juergen Gross
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