Re: [PATCH 1/2] fs/resctrl: Return -EBUSY when per-CLOSID RMIDs are in limbo

[Jinjie Ruan]

在 2026/10/30 2:12, Reinette Chatre 写道:
> Hi Jinjie,
> 
> Thank you very much for these fixes.
> 
> On 10/29/26 1:15 AM, Jinjie Ruan wrote:
>> resctrl_find_free_rmid() returns -ENOSPC when no matching RMID is
>> found. With CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID the target CLOSID may
>> have RMIDs in limbo rather than permanently exhausted.
> 
> Just a quick note about customs around resctrl fs and x86 patches that relate to
> both of these patches:
>   The resctrl patches need to follow the tip requirements documented in
>   Documentation/process/maintainer-tip.rst. Of particular note is the "Changelog"
>   section that needs fixes to be described in a particular format consisting
>   of a context, problem, and solution.

Hi, Reinette

Thanks for the reminder.

>   Apart from this the changelog should avoid narrating the code and instead
>   describe it at a higher level to support understanding without needing to
>   refer to the code and the impact.

I got it.

> 
> Even though the changelog mentions CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID
> it is written for somebody very familiar with the code and it is possible to
> state the impact more clearly without needing to go look where
> CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID is set. I also think that it would be helpful
> to describe all scenarios of the issue to help support the shape of the fix.

Ok, will add them.

> 
> As an example that aims to address all comments above, consider a changelog
> like below:
> 
>   fs/resctrl: Report a temporary RMID shortage per CLOSID
> 
>   An RMID that is freed is held in limbo until its cache occupancy drops
>   below the threshold, after which it returns to the free list. A
>   request that fails because of limbo is thus temporary and worth
>   retrying, which resctrl reports to userspace as -EBUSY. A request
>   that fails because every RMID is in use is permanent and reported as
>   -ENOSPC.
> 
>   Where an RMID is only meaningful together with a CLOSID, a request can
>   only be satisfied by an RMID belonging to the requested CLOSID, so the
>   distinction has to be drawn per CLOSID. Instead it is drawn system
>   wide. -ENOSPC is reported when the free list holds no RMID for the
>   requested CLOSID, even when that CLOSID has RMIDs in limbo. -EBUSY is
>   reported when the free list is empty and another CLOSID has an RMID in
>   limbo, even when the requested CLOSID has none. Userspace is thus
>   told to give up on a shortage that may clear on its own, and to retry
>   one that cannot clear until it removes a group itself.
> 
>   Architectures where the RMID is independent of the CLOSID always match
>   the first entry on the free list, so neither case arises there and only
>   MPAM systems are affected.
> 
>   Draw the distinction from the number of RMIDs the requested CLOSID has
>   in limbo where the RMID depends on the CLOSID, and from the system
>   wide count otherwise. Drop the separate test for an empty free list,
>   which the search handles on its own.

Thank you very much for your example.

> 
>>
>> Check closid_num_dirty_rmid[] for the target CLOSID before returning
>> -ENOSPC, and return -EBUSY so userspace can retry.
>>
>> Cc: James Morse <james.morse@xxxxxxx>
>> Cc: Reinette Chatre <reinette.chatre@xxxxxxxxx>
>> Cc: Babu Moger <babu.moger@xxxxxxx>
>> Fixes: c4c0376eefe1 ("x86/resctrl: Allow RMID allocation to be scoped by CLOSID")
> 
> We should take care here. Even though this "Fixes:" commit added support for
> RMID allocation to be scoped by CLOSID this issue only impacts MPAM systems,
> and MPAM support landed after this commit.
> 
> Do you think this needs a "Cc: stable" tag? Have you heard complaints from

Yes , this need one.

> users or is this just something you noticed? Having a benefit to users may
> be sufficient justification for adding a stable tag. Accuracy of
> "Fixes:" tag is then very important, or the stable tag self can have extra
> information on which kernels need it.
> 
>> Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
> 
> Please re-order the tags based on "Ordering of commit tags" found in
> Documentation/process/maintainer-tip.rst

Thank you very much! I will check it.

> 
>> ---
>>  fs/resctrl/monitor.c | 15 +++++++++++----
>>  1 file changed, 11 insertions(+), 4 deletions(-)
>>
> Patch looks good to me.
> 
> Thank you very much.
> 
> Reinette

-- 
Best regards,
Jinjie