Re: [PATCH v7 4/9] nvme-multipath: add support for latency I/O policy

[Nilay Shroff <nilay@linux.ibm.com>]

On 8/10/26 1:42 PM, John Garry wrote:
> On 09/08/2026 11:07, Nilay Shroff wrote:
>> +    /*
>> +     * If latency > ~1s then ignore this sample to prevent EWMA from being
>> +     * skewed by pathological outliers (multi-second waits, controller
>> +     * timeouts etc.). This keeps path scores representative of normal
>> +     * performance and avoids instability from rare spikes. If such high
>> +     * latency is real, ANA state reporting or keep-alive error counters
>> +     * will mark the path unhealthy and remove it from the head node list,
>> +     * so we safely skip such sample here.
>> +     */
>> +    if (unlikely(latency > NSEC_PER_SEC)) {
>> +        stat->nr_ignored++;
>> +        dev_warn_ratelimited(ns->ctrl->device,
>> +            "ignoring sample with >1s latency (possible controller stall or timeout)\n");
>> +        return;
>> +    }
> 
> JFYI, I gave this series a spin and I see this warn a lot:
> 
> # ./fio_read.sh nvme1n1
> iops: (g=0): rw=read, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T)
> 4096B-4096B, ioengine=libaio, iodepth=100
> ...
> fio-3.38
> Starting 10 processes
> [  130.005374] nvme_mpath_add_sample: 29 callbacks suppressed03h:08m:36s]
> [  130.005387] nvme nvme2: ignoring sample with >1s latency (possible
> controller stall or timeout)
> [  130.009968] nvme nvme2: ignoring sample with >1s latency (possible
> controller stall or timeout)
> [  130.013176] nvme nvme2: ignoring sample with >1s latency (possible
> controller stall or timeout)
> [  130.019576] nvme nvme2: ignoring sample with >1s latency (possible
> controller stall or timeout)
> [  130.022275] nvme nvme2: ignoring sample with >1s latency (possible
> controller stall or timeout)
> [  130.024762] nvme nvme2: ignoring sample with >1s latency (possible
> controller stall or timeout)
> [  130.027268] nvme nvme2: ignoring sample with >1s latency (possible
> controller stall or timeout)
> [  130.130140] nvme nvme2: ignoring sample with >1s latency (possible
> controller stall or timeout)
> [  130.132316] nvme nvme2: ignoring sample with >1s latency (possible
> controller stall or timeout)
> [  130.139279] nvme nvme2: ignoring sample with >1s latency (possible
> controller stall or timeout)
> ^Cbs: 10 (f=10): [R(10)][0.4%][r=35.8MiB/s][r=9171 IOPS][eta 03h:07m:44s]
> fio: terminating on signal 2
> 
> iops: (groupid=0, jobs=10): err= 0: pid=821: Mon Aug 10 08:08:28 2026
>     read: IOPS=9239, BW=36.1MiB/s (37.8MB/s)(1434MiB/39744msec)
>       slat (usec): min=25, max=110836, avg=129.22, stdev=537.18
> 
> Note that I do have many heavy kernel debug options enabled, like kasan and kmemleak, which may influence this.
> 

Thanks for testing. Given that you're running with KASAN, kmemleak, and other
heavy debug options enabled, I think this is expected. Those configurations
can significantly increase I/O completion latency, causing some samples to
exceed the 1-second threshold. The warning is also rate-limited to avoid
flooding the log. That said, still if it's causing a lot of noise in the logs
then we could change it to dev_dbg_ratelimited() from dev_warn_ratelimited().

Thanks,
--Nilay