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SubjectRe: [PATCH bpf-next v2 2/5] nbd: trace sending nbd requests
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On Sat, 2019-04-06 at 20:10 -0400, Josef Bacik wrote:
> On Fri, Apr 05, 2019 at 04:55:03PM -0700, Matt Mullins wrote:
> > This adds a tracepoint that can both observe the nbd request being sent
> > to the server, as well as modify that request , e.g., setting a flag in
> > the request that will cause the server to collect detailed tracing data.
> >
> > The struct request * being handled is included to permit correlation
> > with the block tracepoints.
> >
> > Signed-off-by: Matt Mullins <mmullins@fb.com>
> > ---
> > MAINTAINERS | 1 +
> > drivers/block/nbd.c | 5 ++++
> > include/trace/events/nbd.h | 56 ++++++++++++++++++++++++++++++++++++++
> > 3 files changed, 62 insertions(+)
> > create mode 100644 include/trace/events/nbd.h
> >
> > diff --git a/MAINTAINERS b/MAINTAINERS
> > index e17ebf70b548..6db583d2b0ea 100644
> > --- a/MAINTAINERS
> > +++ b/MAINTAINERS
> > @@ -10737,6 +10737,7 @@ L: linux-block@vger.kernel.org
> > L: nbd@other.debian.org
> > F: Documentation/blockdev/nbd.txt
> > F: drivers/block/nbd.c
> > +F: include/trace/events/nbd.h
> > F: include/uapi/linux/nbd.h
> >
> > NETWORK DROP MONITOR
> > diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> > index 90ba9f4c03f3..7393d04d255c 100644
> > --- a/drivers/block/nbd.c
> > +++ b/drivers/block/nbd.c
> > @@ -44,6 +44,9 @@
> > #include <linux/nbd-netlink.h>
> > #include <net/genetlink.h>
> >
> > +#define CREATE_TRACE_POINTS
> > +#include <trace/events/nbd.h>
> > +
> > static DEFINE_IDR(nbd_index_idr);
> > static DEFINE_MUTEX(nbd_index_mutex);
> > static int nbd_total_devices = 0;
> > @@ -526,6 +529,8 @@ static int nbd_send_cmd(struct nbd_device *nbd, struct nbd_cmd *cmd, int index)
> > handle = nbd_cmd_handle(cmd);
> > memcpy(request.handle, &handle, sizeof(handle));
> >
> > + trace_nbd_send_request(&request, nbd->index, blk_mq_rq_from_pdu(cmd));
> > +
>
> Just use the handle, not the pointer. Thanks,

Assuming you're talking about the struct request *, we are tracing
things through from blk_mq_start_request, which emits tracepoints like

DEFINE_EVENT(block_rq, block_rq_issue,
TP_PROTO(struct request_queue *q, struct request *rq),

We need at least one of the tracepoints to observe a correlation
between NBD handle (in this case, embedded in the struct nbd_request)
and the struct request. That said, I don't actually mind if they're
the hashed kind of pointers, as long as the rest of the block
tracepoints do the same...
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