netdev - Re: [PATCH 01/23] io_uring: shuffle rarely used ctx fields

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Date:   Thu, 20 May 2021 23:46:02 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Song Liu <songliubraving@...com>
Cc:     "io-uring@...r.kernel.org" <io-uring@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
        Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Horst Schirmeier <horst.schirmeier@...dortmund.de>,
        "Franz-B . Tuneke" <franz-bernhard.tuneke@...dortmund.de>,
        Christian Dietrich <stettberger@...ucode.de>
Subject: Re: [PATCH 01/23] io_uring: shuffle rarely used ctx fields

On 5/20/21 10:46 PM, Song Liu wrote:
>> On May 19, 2021, at 7:13 AM, Pavel Begunkov <asml.silence@...il.com> wrote:
>> There is a bunch of scattered around ctx fields that are almost never
>> used, e.g. only on ring exit, plunge them to the end, better locality,
>> better aesthetically.
>>
>> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
>> ---
>> fs/io_uring.c | 36 +++++++++++++++++-------------------
>> 1 file changed, 17 insertions(+), 19 deletions(-)
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 9ac5e278a91e..7e3410ce100a 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -367,9 +367,6 @@ struct io_ring_ctx {
>> 		unsigned		cached_cq_overflow;
>> 		unsigned long		sq_check_overflow;
>>
>> -		/* hashed buffered write serialization */
>> -		struct io_wq_hash	*hash_map;
>> -
>> 		struct list_head	defer_list;
>> 		struct list_head	timeout_list;
>> 		struct list_head	cq_overflow_list;
>> @@ -386,9 +383,6 @@ struct io_ring_ctx {
>>
>> 	struct io_rings	*rings;
>>
>> -	/* Only used for accounting purposes */
>> -	struct mm_struct	*mm_account;
>> -
>> 	const struct cred	*sq_creds;	/* cred used for __io_sq_thread() */
>> 	struct io_sq_data	*sq_data;	/* if using sq thread polling */
>>
>> @@ -409,14 +403,6 @@ struct io_ring_ctx {
>> 	unsigned		nr_user_bufs;
>> 	struct io_mapped_ubuf	**user_bufs;
>>
>> -	struct user_struct	*user;
>> -
>> -	struct completion	ref_comp;
>> -
>> -#if defined(CONFIG_UNIX)
>> -	struct socket		*ring_sock;
>> -#endif
>> -
>> 	struct xarray		io_buffers;
>>
>> 	struct xarray		personalities;
>> @@ -460,12 +446,24 @@ struct io_ring_ctx {
>>
>> 	struct io_restriction		restrictions;
>>
>> -	/* exit task_work */
>> -	struct callback_head		*exit_task_work;
>> -
>> 	/* Keep this last, we don't need it for the fast path */
>> -	struct work_struct		exit_work;
>> -	struct list_head		tctx_list;
>> +	struct {
> 
> Why do we need an anonymous struct here? For cache line alignment?
> Do we need ____cacheline_aligned_in_smp?

Rather as a visual hint considering that most of the field historically
are in structs (____cacheline_aligned_in_smp). Also preparing to
potentially splitting it out of the ctx struct as it grows big. 

First 2-3 patches are not strictly related to bpf and will go separately
earlier, just the set was based on.

>> +		#if defined(CONFIG_UNIX)
>> +			struct socket		*ring_sock;
>> +		#endif
>> +		/* hashed buffered write serialization */
>> +		struct io_wq_hash		*hash_map;
>> +
>> +		/* Only used for accounting purposes */
>> +		struct user_struct		*user;
>> +		struct mm_struct		*mm_account;
>> +
>> +		/* ctx exit and cancelation */
>> +		struct callback_head		*exit_task_work;
>> +		struct work_struct		exit_work;
>> +		struct list_head		tctx_list;
>> +		struct completion		ref_comp;
>> +	};
>> };
>>
>> struct io_uring_task {

-- 
Pavel Begunkov