Messages in this thread | | | Subject | Re: [PATCH] scsi: storvsc: Cap scsi_driver.can_queue to fix a hang issue during boot | From | John Garry <> | Date | Wed, 6 Oct 2021 17:03:09 +0100 |
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On 06/10/2021 16:01, Michael Kelley wrote: > From: John Garry <john.garry@huawei.com> Sent: Wednesday, October 6, 2021 1:17 AM >> >> On 06/10/2021 08:03, Dexuan Cui wrote: >>> After commit ea2f0f77538c, a 416-CPU VM running on Hyper-V hangs during >>> boot because scsi_add_host_with_dma() sets shost->cmd_per_lun to a >>> negative number: >>> 'max_outstanding_req_per_channel' is 352, >>> 'max_sub_channels' is (416 - 1) / 4 = 103, so in storvsc_probe(), >>> scsi_driver.can_queue = 352 * (103 + 1) * (100 - 10) / 100 = 32947, which >>> is bigger than SHRT_MAX (i.e. 32767). >> >> Out of curiosity, are these values realistic? You're capping can_queue >> just because of a data size issue, so, if these values are realistic, >> seems a weak reason. >> > > The calculated value of can_queue is not realistic. The blk-mq layer > caps the number of tags at 10240,
nit: 1024, I think
> so the excessively large value > calculated here didn't definitively break anything, though it can be > poor from a performance tuning standpoint. The algorithm used here > is fairly broken, particularly in VMs with large CPU counts. I have an > effort underway to fix it, but its part of a bigger set of changes to also > do a better job on the perf tuning aspects. > >>> >>> Fix the hang issue by capping scsi_driver.can_queue. >>> >>> Add the below Fixed tag though ea2f0f77538c itself is good. >>> >>> Fixes: ea2f0f77538c ("scsi: core: Cap scsi_host cmd_per_lun at can_queue") >>> Cc: stable@vger.kernel.org >>> Signed-off-by: Dexuan Cui <decui@microsoft.com> >>> --- >>> drivers/scsi/storvsc_drv.c | 10 ++++++++++ >>> 1 file changed, 10 insertions(+) >>> >>> diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c >>> index ebbbc1299c62..ba374908aec2 100644 >>> --- a/drivers/scsi/storvsc_drv.c >>> +++ b/drivers/scsi/storvsc_drv.c >>> @@ -1976,6 +1976,16 @@ static int storvsc_probe(struct hv_device *device, >>> (max_sub_channels + 1) * >>> (100 - ring_avail_percent_lowater) / 100; >>> >>> + /* >>> + * v5.14 (see commit ea2f0f77538c) implicitly requires that >>> + * scsi_driver.can_queue should not exceed SHRT_MAX, otherwise >>> + * scsi_add_host_with_dma() sets shost->cmd_per_lun to a negative >>> + * number (note: the type of the "cmd_per_lun" field is "short"), and >>> + * the system may hang during early boot. >>> + */ >> >> The different data sizes for cmd_per_lun and can_queue are problematic here. >> >> I'd be more inclined to set cmd_per_lun to the same data size as >> can_queue. We did discuss this when ea2f0f77538c was upstreamed >> (actually it was the other way around - setting can_queue to 16b). > > I can see that making can_queue be 16 bits would make sense. > And it also seems that both cmd_per_lun and can_queue should be > unsigned, though I don't the implications of making such a change. > > But in today's world where cmd_per_lun is "short" and can_queue > is "int", ea2f0f77538c seems incorrect to me. The comparison should > be done as "int", not "short", in order to prevent the truncation > problem with can_queue that Dexuan's patch is trying to address.
Yeah, right. I think can_queue values > short_max was considered outside the realms of what is realistic then, hence my sloppy programming.
> The result will always fit in back into the "short" cmd_per_lun since > it is calculating a "min" function. > >> >> Thanks, >> John >> >> >>> + if (scsi_driver.can_queue > SHRT_MAX) >>> + scsi_driver.can_queue = SHRT_MAX; >>> + > > This fix works, but is a more of a temporary hack until I can finish > a larger overhaul of the algorithm.
> But for now, I think the better > fix is for ea2f0f77538c to do the comparison as "int" instead of "short". >
That seems better to me. But Let's wait for other possible opinion.
Thanks, John
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