Re: [RFC PATCH 4/4] KEYS: key_garbage_collector(): process in chunks

[Jarkko Sakkinen]

On Mon, Nov 02, 2026 at 02:15:15PM +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.
> 4. Define KEY_GC_REAP_REQUESTED for preserving full-pass requests across
>    the continuations.
> 
> keygcbench results without the patch:
> 
> unused:   min=413.359 avg=1817.240 max=3221.120 var=1970880.024
> expired:  min=0.002 avg=1360.124 max=2481.990 var=1055093.994
> combined: min=0.002 avg=1542.970 max=3221.120 var=1471557.422
> 
> keygcbench results with the patch:
> 
> unused:   min=0.005 avg=0.126 max=54.173 var=0.946
> expired:  min=0.002 avg=0.210 max=216.470 var=7.894
> combined: min=0.002 avg=0.158 max=216.470 var=3.627
> 
> These results translate to:
> 
> - >99.9% drop in the combined variance.
> - 3.0% 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@xxxxxxxxxx>
 
I left out syzkaller tags (and do so also in the foreseeable future) as
they seem to influence kind of feedback to appear that can only
interfere the topic.

Br, Jarkko