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Date: Tue, 11 Jun 2024 09:55:11 +0200
From: Jesper Dangaard Brouer <hawk@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
"David S. Miller" <davem@...emloft.net>,
Daniel Bristot de Oliveira <bristot@...nel.org>,
Boqun Feng <boqun.feng@...il.com>, Daniel Borkmann <daniel@...earbox.net>,
Eric Dumazet <edumazet@...gle.com>, Frederic Weisbecker
<frederic@...nel.org>, Ingo Molnar <mingo@...hat.com>,
Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>,
Waiman Long <longman@...hat.com>, Will Deacon <will@...nel.org>,
Alexei Starovoitov <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>,
Eduard Zingerman <eddyz87@...il.com>, Hao Luo <haoluo@...gle.com>,
Jiri Olsa <jolsa@...nel.org>, John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>,
Song Liu <song@...nel.org>, Stanislav Fomichev <sdf@...gle.com>,
Toke Høiland-Jørgensen <toke@...hat.com>,
Yonghong Song <yonghong.song@...ux.dev>, bpf@...r.kernel.org
Subject: Re: [PATCH v5 net-next 14/15] net: Reference bpf_redirect_info via
task_struct on PREEMPT_RT.
On 10/06/2024 18.50, Sebastian Andrzej Siewior wrote:
> On 2024-06-07 13:51:25 [+0200], Jesper Dangaard Brouer wrote:
>> The memset can be further optimized as it currently clears 64 bytes, but
>> it only need to clear 40 bytes, see pahole below.
>>
>> Replace memset with something like:
>> memset(&bpf_net_ctx->ri, 0, offsetof(struct bpf_net_context, ri.nh));
>>
>> This is an optimization, because with 64 bytes this result in a rep-stos
>> (repeated string store operation) that on Intel touch CPU-flags (to be
>> IRQ safe) which is slow, while clearing 40 bytes doesn't cause compiler
>> to use this instruction, which is faster. Memset benchmarked with [1]
>
> I've been playing along with this and have to say that "rep stosq" is
> roughly 3x slower vs "movq" for 64 bytes on all x86 I've been looking
> at.
Thanks for confirming "rep stos" is 3x slower for small sizes.
> For gcc the stosq vs movq depends on the CPU settings. The generic uses
> movq up to 40 bytes, skylake uses movq even for 64bytes. clang…
> This could be tuned via -mmemset-strategy=libcall:64:align,rep_8byte:-1:align
>
Cool I didn't know of this tuning. Is this a compiler option?
Where do I change this setting, as I would like to experiment with this
for our prod kernels.
My other finding is, this primarily a kernel compile problem, because
for userspace compiler chooses to use MMX instructions (e.g. movaps
xmmword ptr[rsp], xmm0). The kernel compiler options (-mno-sse -mno-mmx
-mno-sse2 -mno-3dnow -mno-avx) disables this, which aparently changes
the tipping point.
> I folded this into the last two patches:
>
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index d2b4260d9d0be..1588d208f1348 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -744,27 +744,40 @@ struct bpf_redirect_info {
> struct bpf_nh_params nh;
> };
>
> +enum bpf_ctx_init_type {
> + bpf_ctx_ri_init,
> + bpf_ctx_cpu_map_init,
> + bpf_ctx_dev_map_init,
> + bpf_ctx_xsk_map_init,
> +};
> +
> struct bpf_net_context {
> struct bpf_redirect_info ri;
> struct list_head cpu_map_flush_list;
> struct list_head dev_map_flush_list;
> struct list_head xskmap_map_flush_list;
> + unsigned int flags;
Why have yet another flags variable, when we already have two flags in
bpf_redirect_info ?
> };
>
> +static inline bool bpf_net_ctx_need_init(struct bpf_net_context *bpf_net_ctx,
> + enum bpf_ctx_init_type flag)
> +{
> + return !(bpf_net_ctx->flags & (1 << flag));
> +}
> +
> +static inline bool bpf_net_ctx_set_flag(struct bpf_net_context *bpf_net_ctx,
> + enum bpf_ctx_init_type flag)
> +{
> + return bpf_net_ctx->flags |= 1 << flag;
> +}
> +
> static inline struct bpf_net_context *bpf_net_ctx_set(struct bpf_net_context *bpf_net_ctx)
> {
> struct task_struct *tsk = current;
>
> if (tsk->bpf_net_context != NULL)
> return NULL;
> - memset(&bpf_net_ctx->ri, 0, sizeof(bpf_net_ctx->ri));
> -
> - if (IS_ENABLED(CONFIG_BPF_SYSCALL)) {
> - INIT_LIST_HEAD(&bpf_net_ctx->cpu_map_flush_list);
> - INIT_LIST_HEAD(&bpf_net_ctx->dev_map_flush_list);
> - }
> - if (IS_ENABLED(CONFIG_XDP_SOCKETS))
> - INIT_LIST_HEAD(&bpf_net_ctx->xskmap_map_flush_list);
> + bpf_net_ctx->flags = 0;
>
> tsk->bpf_net_context = bpf_net_ctx;
> return bpf_net_ctx;
> @@ -785,6 +798,11 @@ static inline struct bpf_redirect_info *bpf_net_ctx_get_ri(void)
> {
> struct bpf_net_context *bpf_net_ctx = bpf_net_ctx_get();
>
> + if (bpf_net_ctx_need_init(bpf_net_ctx, bpf_ctx_ri_init)) {
> + memset(&bpf_net_ctx->ri, 0, offsetof(struct bpf_net_context, ri.nh));
> + bpf_net_ctx_set_flag(bpf_net_ctx, bpf_ctx_ri_init);
> + }
> +
> return &bpf_net_ctx->ri;
> }
>
> @@ -792,6 +810,11 @@ static inline struct list_head *bpf_net_ctx_get_cpu_map_flush_list(void)
> {
> struct bpf_net_context *bpf_net_ctx = bpf_net_ctx_get();
>
> + if (bpf_net_ctx_need_init(bpf_net_ctx, bpf_ctx_cpu_map_init)) {
> + INIT_LIST_HEAD(&bpf_net_ctx->cpu_map_flush_list);
> + bpf_net_ctx_set_flag(bpf_net_ctx, bpf_ctx_cpu_map_init);
> + }
> +
> return &bpf_net_ctx->cpu_map_flush_list;
> }
>
> @@ -799,6 +822,11 @@ static inline struct list_head *bpf_net_ctx_get_dev_flush_list(void)
> {
> struct bpf_net_context *bpf_net_ctx = bpf_net_ctx_get();
>
> + if (bpf_net_ctx_need_init(bpf_net_ctx, bpf_ctx_dev_map_init)) {
> + INIT_LIST_HEAD(&bpf_net_ctx->dev_map_flush_list);
> + bpf_net_ctx_set_flag(bpf_net_ctx, bpf_ctx_dev_map_init);
> + }
> +
> return &bpf_net_ctx->dev_map_flush_list;
> }
>
> @@ -806,6 +834,11 @@ static inline struct list_head *bpf_net_ctx_get_xskmap_flush_list(void)
> {
> struct bpf_net_context *bpf_net_ctx = bpf_net_ctx_get();
>
> + if (bpf_net_ctx_need_init(bpf_net_ctx, bpf_ctx_xsk_map_init)) {
> + INIT_LIST_HEAD(&bpf_net_ctx->xskmap_map_flush_list);
> + bpf_net_ctx_set_flag(bpf_net_ctx, bpf_ctx_xsk_map_init);
> + }
> +
> return &bpf_net_ctx->xskmap_map_flush_list;
> }
>
>
> Sebastian
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