Re: [PATCH v1 1/3] net: Introduce sk_use_task_frag in struct sock.

[Paolo Abeni <pabeni@redhat.com>]

On Mon, 2022-11-21 at 08:35 -0500, Benjamin Coddington wrote:
> From: Guillaume Nault <gnault@redhat.com>
> 
> Sockets that can be used while recursing into memory reclaim, like
> those used by network block devices and file systems, mustn't use
> current->task_frag: if the current process is already using it, then
> the inner memory reclaim call would corrupt the task_frag structure.
> 
> To avoid this, sk_page_frag() uses ->sk_allocation to detect sockets
> that mustn't use current->task_frag, assuming that those used during
> memory reclaim had their allocation constraints reflected in
> ->sk_allocation.
> 
> This unfortunately doesn't cover all cases: in an attempt to remove all
> usage of GFP_NOFS and GFP_NOIO, sunrpc stopped setting these flags in
> ->sk_allocation, and used memalloc_nofs critical sections instead.
> This breaks the sk_page_frag() heuristic since the allocation
> constraints are now stored in current->flags, which sk_page_frag()
> can't read without risking triggering a cache miss and slowing down
> TCP's fast path.
> 
> This patch creates a new field in struct sock, named sk_use_task_frag,
> which sockets with memory reclaim constraints can set to false if they
> can't safely use current->task_frag. In such cases, sk_page_frag() now
> always returns the socket's page_frag (->sk_frag). The first user is
> sunrpc, which needs to avoid using current->task_frag but can keep
> ->sk_allocation set to GFP_KERNEL otherwise.
> 
> Eventually, it might be possible to simplify sk_page_frag() by only
> testing ->sk_use_task_frag and avoid relying on the ->sk_allocation
> heuristic entirely (assuming other sockets will set ->sk_use_task_frag
> according to their constraints in the future).
> 
> The new ->sk_use_task_frag field is placed in a hole in struct sock and
> belongs to a cache line shared with ->sk_shutdown. Therefore it should
> be hot and shouldn't have negative performance impacts on TCP's fast
> path (sk_shutdown is tested just before the while() loop in
> tcp_sendmsg_locked()).
> 
> Link: https://lore.kernel.org/netdev/b4d8cb09c913d3e34f853736f3f5628abfd7f4b6.1656699567.git.gnault@redhat.com/
> Signed-off-by: Guillaume Nault <gnault@redhat.com>
> ---
>  include/net/sock.h | 11 +++++++++--
>  net/core/sock.c    |  1 +
>  2 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/include/net/sock.h b/include/net/sock.h
> index d08cfe190a78..ffba9e95470d 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -318,6 +318,9 @@ struct sk_filter;
>    *	@sk_stamp: time stamp of last packet received
>    *	@sk_stamp_seq: lock for accessing sk_stamp on 32 bit architectures only
>    *	@sk_tsflags: SO_TIMESTAMPING flags
> +  *	@sk_use_task_frag: allow sk_page_frag() to use current->task_frag.
> +			   Sockets that can be used under memory reclaim should
> +			   set this to false.
>    *	@sk_bind_phc: SO_TIMESTAMPING bind PHC index of PTP virtual clock
>    *	              for timestamping
>    *	@sk_tskey: counter to disambiguate concurrent tstamp requests
> @@ -504,6 +507,7 @@ struct sock {
>  #endif
>  	u16			sk_tsflags;
>  	u8			sk_shutdown;
> +	bool			sk_use_task_frag;
>  	atomic_t		sk_tskey;
>  	atomic_t		sk_zckey;

I think the above should be fine from a data locality PoV, as the used
cacheline should be hot at sk_page_frag_refill() usage time, as
sk_tsflags has been accessed just before.

@Eric, does the above fit with the planned sock fields reordering?

Jakub noted we could use a bitfield here to be future proof for
additional flags addition. I think in this specific case a bool is
preferable, because we actually wont to discourage people to add more
of such flags, and the search for holes (or the bool -> bitflag
conversion) should give to such eventual future changes some additional
thoughts.

Thanks!

Paolo