linux-kernel - Re: [PATCH v4 2/2] io_uring: batch getting pcpu references

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Date:   Sun, 29 Dec 2019 19:33:22 -0800
From:   Brian Gianforcaro <b.gianfo@...il.com>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] io_uring: batch getting pcpu references

On Sat, Dec 28, 2019 at 09:37:35PM +0300, Pavel Begunkov wrote:
> On 28/12/2019 20:03, Jens Axboe wrote:
> > On 12/28/19 4:15 AM, Pavel Begunkov wrote:
> >> On 28/12/2019 14:13, Pavel Begunkov wrote:
> >>> percpu_ref_tryget() has its own overhead. Instead getting a reference
> >>> for each request, grab a bunch once per io_submit_sqes().
> >>>
> >>> ~5% throughput boost for a "submit and wait 128 nops" benchmark.
> >>>
> >>> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
> >>> ---
> >>>  fs/io_uring.c | 26 +++++++++++++++++---------
> >>>  1 file changed, 17 insertions(+), 9 deletions(-)
> >>>
> >>> diff --git a/fs/io_uring.c b/fs/io_uring.c
> >>> index 7fc1158bf9a4..404946080e86 100644
> >>> --- a/fs/io_uring.c
> >>> +++ b/fs/io_uring.c
> >>> @@ -1080,9 +1080,6 @@ static struct io_kiocb *io_get_req(struct io_ring_ctx *ctx,
> >>>  	gfp_t gfp = GFP_KERNEL | __GFP_NOWARN;
> >>>  	struct io_kiocb *req;
> >>>  
> >>> -	if (!percpu_ref_tryget(&ctx->refs))
> >>> -		return NULL;
> >>> -
> >>>  	if (!state) {
> >>>  		req = kmem_cache_alloc(req_cachep, gfp);
> >>>  		if (unlikely(!req))
> >>> @@ -1141,6 +1138,14 @@ static void io_free_req_many(struct io_ring_ctx *ctx, void **reqs, int *nr)
> >>>  	}
> >>>  }
> >>>  
> >>> +static void __io_req_free_empty(struct io_kiocb *req)
> >>
> >> If anybody have better naming (or a better approach at all), I'm all ears.
> > 
> > __io_req_do_free()?
> 
> Not quite clear what's the difference with __io_req_free() then
> 
> > 
> > I think that's better than the empty, not quite sure what that means.
> 
> Probably, so. It was kind of "request without a bound sqe".
> Does io_free_{hollow,empty}_req() sound better?

Given your description, perhaps io_free_unbound_req() makes sense?