Message-ID: <6892bb46-e2eb-4373-9ac0-6c43eca78b8e@gmail.com>
Date: Thu, 24 Jul 2025 13:46:08 +0300
From: Tariq Toukan <ttoukan.linux@...il.com>
To: Jakub Kicinski <kuba@...nel.org>, Tariq Toukan <tariqt@...dia.com>
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Subject: Re: [PATCH net-next 0/5] Expose grace period delay for devlink health
 reporter



On 19/07/2025 3:47, Jakub Kicinski wrote:
> On Thu, 17 Jul 2025 19:07:17 +0300 Tariq Toukan wrote:
>> Currently, the devlink health reporter initiates the grace period
>> immediately after recovering an error, which blocks further recovery
>> attempts until the grace period concludes. Since additional errors
>> are not generally expected during this short interval, any new error
>> reported during the grace period is not only rejected but also causes
>> the reporter to enter an error state that requires manual intervention.
>>
>> This approach poses a problem in scenarios where a single root cause
>> triggers multiple related errors in quick succession - for example,
>> a PCI issue affecting multiple hardware queues. Because these errors
>> are closely related and occur rapidly, it is more effective to handle
>> them together rather than handling only the first one reported and
>> blocking any subsequent recovery attempts. Furthermore, setting the
>> reporter to an error state in this context can be misleading, as these
>> multiple errors are manifestations of a single underlying issue, making
>> it unlike the general case where additional errors are not expected
>> during the grace period.
>>
>> To resolve this, introduce a configurable grace period delay attribute
>> to the devlink health reporter. This delay starts when the first error
>> is recovered and lasts for a user-defined duration. Once this grace
>> period delay expires, the actual grace period begins. After the grace
>> period ends, a new reported error will start the same flow again.
>>
>> Timeline summary:
>>
>> ----|--------|------------------------------/----------------------/--
>> error is  error is    grace period delay          grace period
>> reported  recovered  (recoveries allowed)     (recoveries blocked)
>>
>> With grace period delay, create a time window during which recovery
>> attempts are permitted, allowing all reported errors to be handled
>> sequentially before the grace period starts. Once the grace period
>> begins, it prevents any further error recoveries until it ends.
> 
> We are rate limiting recoveries, the "networking solution" to the
> problem you're describing would be to introduce a burst size.
> Some kind of poor man's token bucket filter.
> 
> Could you say more about what designs were considered and why this
> one was chosen?
> 

Please see below.
If no more comments, I'll add the below to the cover letter and re-spin.

Regards,
Tariq

Design alternatives considered:

1. Recover all queues upon any error:
    A brute-force approach that recovers all queues on any error.
    While simple, it is overly aggressive and disrupts unaffected queues
    unnecessarily. Also, because this is handled entirely within the
    driver, it leads to a driver-specific implementation rather than a
    generic one.

2. Per-queue reporter:
    This design would isolate recovery handling per SQ or RQ, effectively
    removing interdependencies between queues. While conceptually clean,
    it introduces significant scalability challenges as the number of
    queues grows, as well as synchronization challenges across multiple
    reporters.

3. Error aggregation with delayed handling:
    Errors arriving during the grace period are saved and processed after
    it ends. While addressing the issue of related errors whose recovery
    is aborted as grace period started, this adds complexity due to
    synchronization needs and contradicts the assumption that no errors
    should occur during a healthy system’s grace period. Also, this
    breaks the important role of grace period in preventing an infinite
    loop of immediate error detection following recovery. In such cases
    we want to stop.

4. Allowing a fixed burst of errors before starting grace period:
    Allows a set number of recoveries before the grace period begins.
    However, it also requires limiting the error reporting window.
    To keep the design simple, the burst threshold becomes redundant.

The grace period delay design was chosen for its simplicity and
precision in addressing the problem at hand. It effectively captures
the temporal correlation of related errors and aligns with the original
intent of the grace period as a stabilization window where further
errors are unexpected, and if they do occur, they indicate an abnormal
system state.

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