Re: [PATCH v6] KEYS: key_garbage_collector(): process in chunks

[Jarkko Sakkinen <jarkko@kernel.org>]

On Fri, Oct 30, 2026 at 06:09:23PM +0200, Jarkko Sakkinen wrote:
> key_garbage_collector() has a design that is not resistant enough for
> workqueue lockups. The duration scales up relative to the number of items
> in the serial tree, which causes a reoccuring theme of being guilty of
> causing workqueue lockups.
> 
> Resolve the problem by processing in chunks:
> 
> 1. Declare a static variable key_gc_work, which owns the garbage collection
>    in-flight state, and key_schedule_gc_work() for re-scheduling the work.
> 2. Define KEY_GC_CHUNK_SIZE, set to 64 keys, as the bound for the number
>    of items scanned and destroyed.
> 3. Define KEY_GC_DESTROYING to mark resume to destruction without repeating
>    the RCU grace period. Drain the graveyard before advancing key-type
>    reaping.
> 4. Define KEY_GC_REAP_REQUESTED for preserving full-pass requests across
>    the continuations.
> 
> keygcbench results without the patch:
> 
> unused:   min=798.785 avg=4181.989 max=7565.193 var=11446068.927
> expired:  min=0.003 avg=2404.971 max=4111.065 var=3061018.501
> combined: min=0.003 avg=3115.778 max=7565.193 var=7172908.943
> 
> keygcbench results with the patch:
> 
> unused:   min=0.005 avg=0.125 max=79.590 var=1.088
> expired:  min=0.002 avg=0.196 max=236.207 var=8.121
> combined: min=0.002 avg=0.154 max=236.207 var=3.912
> 
> These results translate to:
> 
> - 93% drop in the combined variance.
> - 2.2% increase in the total task time.
> 
> The value 64 for KEY_GC_CHUNK_SIZE was cherry-picked from the set {1, 2,
> ..., 1024} by running the benchmark for each chunk size three times.
> 
> At the far left variance is 97% lower but at the same time total time
> grows by 133%, when compared to the selected chunk size. The size was
> picked based on getting a reasonable variance with a minimal throughput
> decline.
> 
> Fixes: 0c061b5707ab ("KEYS: Correctly destroy key payloads when their keytype is removed")
> Link: https://git.kernel.org/pub/scm/linux/kernel/git/jarkko/linux-tpmdd-test.git/tree/package/keygcbench
> Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

Interesting :-) It has been idential other than block size change
for v4, v5 and v6 and updating benchmark result but this is the
first version with Sashiko feedback.

Looking into it...

Br, Jarkko