linux-kernel - Re: [PATCH v4 1/2] nvme: add tracepoint for nvme_setup

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Date:   Mon, 22 Jan 2018 16:53:56 +0100
From:   Johannes Thumshirn <jthumshirn@...e.de>
To:     Christoph Hellwig <hch@....de>
Cc:     Sagi Grimberg <sagi@...mberg.me>,
        Keith Busch <keith.busch@...el.com>,
        Linux Kernel Mailinglist <linux-kernel@...r.kernel.org>,
        Hannes Reinecke <hare@...e.de>,
        Linux NVMe Mailinglist <linux-nvme@...ts.infradead.org>,
        "Martin K . Petersen" <martin.petersen@...cle.com>
Subject: Re: [PATCH v4 1/2] nvme: add tracepoint for nvme_setup_cmd

On Mon, Jan 22, 2018 at 04:23:01PM +0100, Christoph Hellwig wrote:
> On Fri, Jan 19, 2018 at 03:18:18PM +0100, Johannes Thumshirn wrote:
> > Signed-off-by: Johannes Thumshirn <jthumshirn@...e.de>
> > Reviewed-by: Hannes Reinecke <hare@...e.de>
> > Reviewed-by: Martin K. Petersen <martin.petersen@...cle.com>
> > Reviewed-by: Keith Busch <keith.busch@...el.com>
> 
> This could really use a changelog explaining what you're tracing,
> and most importantly why.

OK.
 
> > +struct rw_cmd {
> > +	__le64 slba;
> > +	__le16 length;
> > +	__le16 control;
> > +	__le32 dsmgmt;
> > +	__le32 reftag;
> > +	__le16 apptag;
> > +	__le16 appmask;
> > +};
> > +
> > +struct dsm_cmd {
> > +	__le32 nr;
> > +	__le32 attributes;
> > +	__u32 rsvd12[4];
> > +};
> 
> Why do we need all these different defintions?  Just use
> cdw2/3/10/11/12/13 for the fields and decode those __le32 on
> a per-command basis where needed.

Yup.

> > +const char *nvme_trace_parse_cmd(struct trace_seq *p, bool admin,
> > +				 u8 opcode, __le32 *cdw10)
> > +{
> > +	if (admin) {
> > +		switch (opcode) {
> > +		case nvme_admin_create_sq:
> > +			return nvme_trace_create_sq(p, cdw10);
> > +		case nvme_admin_create_cq:
> > +			return nvme_trace_create_cq(p, cdw10);
> > +		case nvme_admin_identify:
> > +			return nvme_trace_admin_identify(p, cdw10);
> > +		default:
> > +			return nvme_trace_common(p, cdw10);
> > +		}
> > +	} else {
> > +		switch (opcode) {
> > +		case nvme_cmd_read:
> > +		case nvme_cmd_write:
> > +		case nvme_cmd_write_zeroes:
> > +			return nvme_trace_read_write(p, cdw10);
> > +		case nvme_cmd_dsm:
> > +			return nvme_trace_dsm(p, cdw10);
> > +		default:
> > +			return nvme_trace_common(p, cdw10);
> > +		}
> > +	}
> > +}
> 
> Wouldn't it be easier to have separate tracepoints for admin vs
> I/O commands?  Especially as people might often want to trace
> only one or the other.

Yes and no. I personally like to have the big hammer when tracing customer
problems and filter out maunally later. I initially had a tracepoint for each
of nvme_setup_flush(), nvme_setup_discard(), nvme_setup_rw() but decided it
was too fine grained.

nvme_setup_cmd() has the nice side effect that all commands, including
userspace passtrough commands must pass it. This was extremely helpful in the
customer bug which inspired me to implement this tracepoint.

Martin, Keith, Sagi, Hannes, any preferences here?

	Johannes
-- 
Johannes Thumshirn                                          Storage
jthumshirn@...e.de                                +49 911 74053 689
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
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