Subject: Re: [PATCH net-next 1/3] net: remove napi_hash_del() from driver-facing API

From: Eric Dumazet <[email-masked]>
Date: Tue, 24 Nov 2020 19:00:50 +0100

On 9/9/20 7:37 PM, Jakub Kicinski wrote:
> We allow drivers to call napi_hash_del() before calling
> netif_napi_del() to batch RCU grace periods. This makes
> the API asymmetric and leaks internal implementation details.
> Soon we will want the grace period to protect more than just
> the NAPI hash table.
> 
> Restructure the API and have drivers call a new function -
> __netif_napi_del() if they want to take care of RCU waits.
> 
> Note that only core was checking the return status from
> napi_hash_del() so the new helper does not report if the
> NAPI was actually deleted.
> 
> Some notes on driver oddness:
>  - veth observed the grace period before calling netif_napi_del()
>    but that should not matter
>  - myri10ge observed normal RCU flavor
>  - bnx2x and enic did not actually observe the grace period
>    (unless they did so implicitly)
>  - virtio_net and enic only unhashed Rx NAPIs
> 
> The last two points seem to indicate that the calls to
> napi_hash_del() were a left over rather than an optimization.
> Regardless, it's easy enough to correct them.
> 
> This patch may introduce extra synchronize_net() calls for
> interfaces which set NAPI_STATE_NO_BUSY_POLL and depend on
> free_netdev() to call netif_napi_del(). This seems inevitable
> since we want to use RCU for netpoll dev->napi_list traversal,
> and almost no drivers set IFF_DISABLE_NETPOLL.
> 
> Signed-off-by: Jakub Kicinski <[email-masked]>

After this patch, gro_cells_destroy() became damn slow
on hosts with a lot of cores.

After your change, we have one additional synchronize_net() per cpu as
you stated in your changelog.

gro_cells_init() is setting NAPI_STATE_NO_BUSY_POLL, and this was enough
to not have one synchronize_net() call per netif_napi_del()

I will test something like :
I am not yet convinced the synchronize_net() is needed, since these
NAPI structs are not involved in busy polling.

diff --git a/net/core/gro_cells.c b/net/core/gro_cells.c
index e095fb871d9120787bfdf62149f4d82e0e3b0a51..8cfa6ce0738977290cc9f76a3f5daa617308e107 100644
--- a/net/core/gro_cells.c
+++ b/net/core/gro_cells.c
@@ -99,9 +99,10 @@ void gro_cells_destroy(struct gro_cells *gcells)
                struct gro_cell *cell = per_cpu_ptr(gcells->cells, i);
 
                napi_disable(&cell->napi);
-               netif_napi_del(&cell->napi);
+               __netif_napi_del(&cell->napi);
                __skb_queue_purge(&cell->napi_skbs);
        }
+       synchronize_net();
        free_percpu(gcells->cells);
        gcells->cells = NULL;
 }