Re: [PATCH v8 3/9] scsi: core: Call .eh_prepare_resubmit() before resubmitting

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On 8/13/23 19:41, Damien Le Moal wrote:
But at the very least, may be mention in the commit message that you also add
the unit tests associated with the change ?

Hi Damien,

I will mention this in the patch description when I repost this patch.

Another remark: we'll need this sorting only for devices that are zoned and do
not need zone write locking. For other cases (most cases in fact), we don't and
this could have some performance impact (e.g. fast RAID systems). Is there a way
to have this eh_prepare_resubmit to do nothing for regular devices to avoid that ?

The common case is that all commands passed to the SCSI error handler
are associated with the same ULD. For this case list_sort() iterates
once over the list with commands that have to be resubmitted because
all eh_prepare_resubmit pointers are identical. The code introduced
in the next patch requires O(n^2) time with n the number of commands
passed to the error handler. I expect this time to be smaller than the
time needed to wake up the error handler if n < 100. Waking up the
error handler involves expediting the grace period (call_rcu_hurry())
and a context switch. I expect that these two mechanisms combined will
take more time than the list_sort() call if the number of commands that
failed is below 100. In other words, I don't think that
scsi_call_prepare_resubmit() will slow down error handling measurably
for the cases we care about.

Do you perhaps want me to change the code in the next patch such that
it takes O(n) time instead of O(n^2) time in case no zoned devices are
involved?

Thanks,

Bart.




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