Re: [PATCH bpf-next v2 00/26] Resilient Queued Spin Lock

Peter Zijlstra
2025-02-10T09:38:49Z

On Thu, Feb 06, 2025 at 02:54:08AM -0800, Kumar Kartikeya Dwivedi wrote:

> Deadlock Detection
> ~~~~~~~~~~~~~~~~~~
> We handle two cases of deadlocks: AA deadlocks (attempts to acquire the
> same lock again), and ABBA deadlocks (attempts to acquire two locks in
> the opposite order from two distinct threads). Variants of ABBA
> deadlocks may be encountered with more than two locks being held in the
> incorrect order. These are not diagnosed explicitly, as they reduce to
> ABBA deadlocks.
>
> Deadlock detection is triggered immediately when beginning the waiting
> loop of a lock slow path.
>
> While timeouts ensure that any waiting loops in the locking slow path
> terminate and return to the caller, it can be excessively long in some
> situations. While the default timeout is short (0.5s), a stall for this
> duration inside the kernel can set off alerts for latency-critical
> services with strict SLOs. Ideally, the kernel should recover from an
> undesired state of the lock as soon as possible.
>
> A multi-step strategy is used to recover the kernel from waiting loops
> in the locking algorithm which may fail to terminate in a bounded amount
> of time.
>
> * Each CPU maintains a table of held locks. Entries are inserted and
> removed upon entry into lock, and exit from unlock, respectively.
> * Deadlock detection for AA locks is thus simple: we have an AA
> deadlock if we find a held lock entry for the lock we’re attempting
> to acquire on the same CPU.
> * During deadlock detection for ABBA, we search through the tables of
> all other CPUs to find situations where we are holding a lock the
> remote CPU is attempting to acquire, and they are holding a lock we
> are attempting to acquire. Upon encountering such a condition, we
> report an ABBA deadlock.
> * We divide the duration between entry time point into the waiting loop
> and the timeout time point into intervals of 1 ms, and perform
> deadlock detection until timeout happens. Upon entry into the slow
> path, and then completion of each 1 ms interval, we perform detection
> of both AA and ABBA deadlocks. In the event that deadlock detection
> yields a positive result, the recovery happens sooner than the
> timeout. Otherwise, it happens as a last resort upon completion of
> the timeout.
>
> Timeouts
> ~~~~~~~~
> Timeouts act as final line of defense against stalls for waiting loops.
> The ‘ktime_get_mono_fast_ns’ function is used to poll for the current
> time, and it is compared to the timestamp indicating the end time in the
> waiter loop. Each waiting loop is instrumented to check an extra
> condition using a macro. Internally, the macro implementation amortizes
> the checking of the timeout to avoid sampling the clock in every
> iteration. Precisely, the timeout checks are invoked every 64k
> iterations.
>
> Recovery
> ~~~~~~~~

I'm probably bad at reading, but I failed to find anything that
explained how you recover from a deadlock.

Do you force unload the BPF program?