Subject: Re: [PATCH bpf-next v3 1/5] bpf: Support chain calling multiple BPF programs after each other

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Tue, 08 Oct 2019 10:07:46 +0200

Alexei Starovoitov <[email-masked]> writes:

> On Mon, Oct 07, 2019 at 07:20:36PM +0200, Toke Høiland-Jørgensen wrote:
>> From: Toke Høiland-Jørgensen <[email-masked]>
>> 
>> This adds support for wrapping eBPF program dispatch in chain calling
>> logic. The code injection is controlled by a flag at program load time; if
>> the flag is set, the BPF program will carry a flag bit that changes the
>> program dispatch logic to wrap it in a chain call loop.
>> 
>> Ideally, it shouldn't be necessary to set the flag on program load time,
>> but rather inject the calls when a chain call program is first loaded. The
>> allocation logic sets the whole of struct bpf_prog to be read-only memory,
>> so it can't immediately be modified, but conceivably we could just unlock
>> the first page of the struct and flip the bit when a chain call program is
>> first attached.
>> 
>> Signed-off-by: Toke Høiland-Jørgensen <[email-masked]>
>> ---
>>  include/linux/bpf.h      |    3 +++
>>  include/linux/filter.h   |   34 ++++++++++++++++++++++++++++++++--
>>  include/uapi/linux/bpf.h |    6 ++++++
>>  kernel/bpf/core.c        |    6 ++++++
>>  kernel/bpf/syscall.c     |    4 +++-
>>  5 files changed, 50 insertions(+), 3 deletions(-)
>> 
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> index 5b9d22338606..13e5f38cf5c6 100644
>> --- a/include/linux/bpf.h
>> +++ b/include/linux/bpf.h
>> @@ -365,6 +365,8 @@ struct bpf_prog_stats {
>>  	struct u64_stats_sync syncp;
>>  };
>>  
>> +#define BPF_NUM_CHAIN_SLOTS 8
>> +
>>  struct bpf_prog_aux {
>>  	atomic_t refcnt;
>>  	u32 used_map_cnt;
>> @@ -383,6 +385,7 @@ struct bpf_prog_aux {
>>  	struct list_head ksym_lnode;
>>  	const struct bpf_prog_ops *ops;
>>  	struct bpf_map **used_maps;
>> +	struct bpf_prog *chain_progs[BPF_NUM_CHAIN_SLOTS];
>>  	struct bpf_prog *prog;
>>  	struct user_struct *user;
>>  	u64 load_time; /* ns since boottime */
>> diff --git a/include/linux/filter.h b/include/linux/filter.h
>> index 2ce57645f3cd..3d1e4991e61d 100644
>> --- a/include/linux/filter.h
>> +++ b/include/linux/filter.h
>> @@ -21,6 +21,7 @@
>>  #include <linux/kallsyms.h>
>>  #include <linux/if_vlan.h>
>>  #include <linux/vmalloc.h>
>> +#include <linux/nospec.h>
>>  
>>  #include <net/sch_generic.h>
>>  
>> @@ -528,6 +529,7 @@ struct bpf_prog {
>>  				is_func:1,	/* program is a bpf function */
>>  				kprobe_override:1, /* Do we override a kprobe? */
>>  				has_callchain_buf:1, /* callchain buffer allocated? */
>> +				chain_calls:1, /* should this use the chain_call wrapper */
>>  				enforce_expected_attach_type:1; /* Enforce expected_attach_type checking at attach time */
>>  	enum bpf_prog_type	type;		/* Type of BPF program */
>>  	enum bpf_attach_type	expected_attach_type; /* For some prog types */
>> @@ -551,6 +553,30 @@ struct sk_filter {
>>  	struct bpf_prog	*prog;
>>  };
>>  
>> +#define BPF_MAX_CHAIN_CALLS 32
>> +static __always_inline unsigned int do_chain_calls(const struct bpf_prog *prog,
>> +						   const void *ctx)
>> +{
>> +	int i = BPF_MAX_CHAIN_CALLS;
>> +	int idx;
>> +	u32 ret;
>> +
>> +	do {
>> +		ret = (*(prog)->bpf_func)(ctx, prog->insnsi);
>
> This breaks program stats.

Oh, right, silly me. Will fix.

>> +
>> +		if (ret + 1 >= BPF_NUM_CHAIN_SLOTS) {
>> +			prog = prog->aux->chain_progs[0];
>> +			continue;
>> +		}
>> +		idx = ret + 1;
>> +		idx = array_index_nospec(idx, BPF_NUM_CHAIN_SLOTS);
>> +
>> +		prog = prog->aux->chain_progs[idx] ?: prog->aux->chain_progs[0];
>> +	} while (prog && --i);
>> +
>> +	return ret;
>> +}
>> +
>>  DECLARE_STATIC_KEY_FALSE(bpf_stats_enabled_key);
>>  
>>  #define BPF_PROG_RUN(prog, ctx)	({				\
>> @@ -559,14 +585,18 @@ DECLARE_STATIC_KEY_FALSE(bpf_stats_enabled_key);
>>  	if (static_branch_unlikely(&bpf_stats_enabled_key)) {	\
>>  		struct bpf_prog_stats *stats;			\
>>  		u64 start = sched_clock();			\
>> -		ret = (*(prog)->bpf_func)(ctx, (prog)->insnsi);	\
>> +		ret = prog->chain_calls ?			\
>> +			do_chain_calls(prog, ctx) :			\
>> +			 (*(prog)->bpf_func)(ctx, (prog)->insnsi);	\
>
> I thought you agreed on 'no performance regressions' rule?

As I wrote in the cover letter I could not measurable a performance
impact from this, even with the simplest possible XDP program (where
program setup time has the largest impact).

This was the performance before/after patch (also in the cover letter):

Before patch (XDP DROP program):  31.5 Mpps
After patch (XDP DROP program):   32.0 Mpps

So actually this *increases* performance ;)
(Or rather, the difference is within the measurement uncertainty on my
system).

-Toke