On Mon, May 18, 2026 at 07:31:45AM -0600, Jens Axboe wrote:
> Why not just issue the trace points? Then there's close to zero
> overhead, rather than needing to need added counters for this, and the
> kernel to keep track. If you just issue the get/put tag kind of traces,
> then userspace can keep track. That's what blktrace has done for decades
> for things like inflight/queue depth accounting.
>
> IOW, seems to me, this could be done with basically zero kernel
> additions outside of perhaps a trace point or two.
Hi Jens,
Thanks for taking a look.
You make a completely fair point.
I agree that pushing the accounting to userspace is the right approach,
especially given the proposed hard-coded tracepoint. For example, with
bpftrace(8):
# bpftrace -e 'tracepoint:block:block_rq_tag_wait { @tag_waits[cpu] = count();
}'
Attaching 1 probe...
^C
@tag_waits[4]: 12
@tag_waits[12]: 87
I will drop Patch 2 from this series, in the next iteration.
Kind regards,
--
Aaron Tomlin
