Re: [PATCH v6 09/13] zram: split zcomp into separate R/W streams

[Hillf Danton <hdanton@sina.com>]

On Wed, 21 Oct 2026 15:53:21 +0800 Hillf Danton wrote:
>On Wed, 21 Oct 2026 13:56:17 +0900 Sergey Senozhatsky wrote:
>>On (26/10/21 12:23), Hillf Danton wrote:
>>> On Wed, 21 Oct 2026 11:42:19 +0900 Sergey Senozhatsky wrote:
>>> > Split struct zcomp_strm into separate compression (Write zcomp_cstrm)
>>> > and decompression (Read zcomp_dstrm) streams, and split zcomp_ops
>>> > context creation/destruction into create_cctx/destroy_cctx and
>>> > optional create_dctx/destroy_dctx callbacks.
>>> > 
>>> > This decouples Read (decompression) and Write (compression) paths so
>>> > they no longer contend on the same per-CPU stream mutex (Writer vs
>>> > Reader priority inversion).  Currently, a (low priority) Writer that
>>> > holds the stream mutex can be preempted by a (higher priority) Reader
>>> > on the same CPU, which then blocks on that mutex until the Writer gets
>>> > to run again and releases it, which can take a long time when there
>>> > are other runnable tasks on that CPU.
>>> >
>>> Given lock like mutex, priority inversion happens from Monday to Friday,
>>> but I wonder if you are inventing some wheels in 2026 if proxy excution
>>> works for you.
>>
>> Reader-Writer split is a foundation for further improvements (like
>> singleton recompression stream and so on).
>>
> But that split is not needed at all if PE works, yes?
>
Because PE is good at handling complex priority hiarchy. In a simple case,
task1(priority=1) is the owner of lock1 with two waiters, task2(priority=2)
and task3(priority=3), while task3 is the owner of lock2 also with two waiters,
task4(priority=5) and task5(priority=7).

When the number of locks and tasks grows, I have increasing difficulty even
describing the lock chain that you in combination with Brian are unable to
handle. That is why in general PE is prefered over whatever wheels you can create.