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Date: Tue, 21 May 2019 17:55:06 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Alexei Starovoitov <ast@...nel.org>
Cc: davem@...emloft.net, Daniel Borkmann <daniel@...earbox.net>,
Networking <netdev@...r.kernel.org>, bpf@...r.kernel.org,
Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next 3/3] bpf: convert explored_states to hash table
On Tue, May 21, 2019 at 4:08 PM Alexei Starovoitov <ast@...nel.org> wrote:
>
> All prune points inside a callee bpf function most likely will have
> different callsites. For example, if function foo() is called from
> two callsites the half of explored states in all prune points in foo()
> will be useless for subsequent walking of one of those callsites.
> Fortunately explored_states pruning heuristics keeps the number of states
> per prune point small, but walking these states is still a waste of cpu
> time when the callsite of the current state is different from the callsite
> of the explored state.
>
> To improve pruning logic convert explored_states into hash table and
> use simple insn_idx ^ callsite hash to select hash bucket.
> This optimization has no effect on programs without bpf2bpf calls
> and drastically improves programs with calls.
> In the later case it reduces total memory consumption in 1M scale tests
> by almost 3 times (peak_states drops from 5752 to 2016).
>
> Care should be taken when comparing the states for equivalency.
> Since the same hash bucket can now contain states with different indices
> the insn_idx has to be part of verifier_state and compared.
>
> Different hash table sizes and different hash functions were explored,
> but the results were not significantly better vs this patch.
> They can be improved in the future.
>
> Hit/miss heuristic is not counting index miscompare as a miss.
> Otherwise verifier stats become unstable when experimenting
> with different hash functions.
>
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> ---
> include/linux/bpf_verifier.h | 1 +
> kernel/bpf/verifier.c | 23 ++++++++++++++++++-----
> 2 files changed, 19 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> index 02bba09a0ea1..405b502283c5 100644
> --- a/include/linux/bpf_verifier.h
> +++ b/include/linux/bpf_verifier.h
> @@ -187,6 +187,7 @@ struct bpf_func_state {
> struct bpf_verifier_state {
> /* call stack tracking */
> struct bpf_func_state *frame[MAX_CALL_FRAMES];
> + u32 insn_idx;
> u32 curframe;
> u32 active_spin_lock;
> bool speculative;
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 89097a4b1bf3..082f6eefb1c4 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -5435,11 +5435,19 @@ enum {
> BRANCH = 2,
> };
>
> +static u32 state_htab_size(struct bpf_verifier_env *env)
maybe mark it as inline function? it's called pretty heavily.
> +{
> + return env->prog->len;
> +}
> +
> static struct bpf_verifier_state_list **explored_state(
> struct bpf_verifier_env *env,
> int idx)
> {
> - return &env->explored_states[idx];
> + struct bpf_verifier_state *cur = env->cur_state;
> + struct bpf_func_state *state = cur->frame[cur->curframe];
> +
> + return &env->explored_states[(idx ^ state->callsite) % state_htab_size(env)];
% is slow, see [1] for faster alternative.
Alternatively, if you can make sure that hash size is power of two,
then multiplicative Fibonacci hashing is preferred ([2]).
[1] https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/
[2] https://probablydance.com/2018/06/16/fibonacci-hashing-the-optimization-that-the-world-forgot-or-a-better-alternative-to-integer-modulo/
> }
>
> static void init_explored_state(struct bpf_verifier_env *env, int idx)
> @@ -6018,7 +6026,8 @@ static void clean_live_states(struct bpf_verifier_env *env, int insn,
>
> sl = *explored_state(env, insn);
> while (sl) {
> - if (sl->state.curframe != cur->curframe)
> + if (sl->state.insn_idx != insn ||
> + sl->state.curframe != cur->curframe)
> goto next;
> for (i = 0; i <= cur->curframe; i++)
> if (sl->state.frame[i]->callsite != cur->frame[i]->callsite)
> @@ -6384,6 +6393,9 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
> clean_live_states(env, insn_idx, cur);
>
> while (sl) {
> + states_cnt++;
> + if (sl->state.insn_idx != insn_idx)
> + goto next;
Shouldn't this be checked inside states_equal? Or you are trying to
avoid function call overhead? If the latter is the case, then you
should probably compare curframe as well here?
> if (states_equal(env, &sl->state, cur)) {
> sl->hit_cnt++;
> /* reached equivalent register/stack state,
> @@ -6401,7 +6413,6 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
> return err;
> return 1;
> }
> - states_cnt++;
> sl->miss_cnt++;
> /* heuristic to determine whether this state is beneficial
> * to keep checking from state equivalence point of view.
> @@ -6428,6 +6439,7 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
> sl = *pprev;
> continue;
> }
> +next:
> pprev = &sl->next;
> sl = *pprev;
> }
> @@ -6459,6 +6471,7 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
> kfree(new_sl);
> return err;
> }
> + new->insn_idx = insn_idx;
> new_sl->next = *explored_state(env, insn_idx);
> *explored_state(env, insn_idx) = new_sl;
> /* connect new state to parentage chain. Current frame needs all
> @@ -8138,7 +8151,7 @@ static void free_states(struct bpf_verifier_env *env)
> if (!env->explored_states)
> return;
>
> - for (i = 0; i < env->prog->len; i++) {
> + for (i = 0; i < state_htab_size(env); i++) {
> sl = env->explored_states[i];
>
> while (sl) {
> @@ -8246,7 +8259,7 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr,
> goto skip_full_check;
> }
>
> - env->explored_states = kvcalloc(env->prog->len,
> + env->explored_states = kvcalloc(state_htab_size(env),
> sizeof(struct bpf_verifier_state_list *),
> GFP_USER);
> ret = -ENOMEM;
> --
> 2.20.0
>
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