Re: [PATCH bpf-next v3] bpf: Do not allocate percpu memory at init stage

[Yonghong Song <yonghong.song@linux.dev>]

On 11/13/23 4:42 AM, Hou Tao wrote:
> Hi,
>
> On 11/11/2023 9:39 AM, Yonghong Song wrote:
>> Kirill Shutemov reported significant percpu memory consumption increase after
>> booting in 288-cpu VM ([1]) due to commit 41a5db8d8161 ("bpf: Add support for
>> non-fix-size percpu mem allocation"). The percpu memory consumption is
>> increased from 111MB to 969MB. The number is from /proc/meminfo.
>>
>> I tried to reproduce the issue with my local VM which at most supports upto
>> 255 cpus. With 252 cpus, without the above commit, the percpu memory
>> consumption immediately after boot is 57MB while with the above commit the
>> percpu memory consumption is 231MB.
>>
>> This is not good since so far percpu memory from bpf memory allocator is not
>> widely used yet. Let us change pre-allocation in init stage to on-demand
>> allocation when verifier detects there is a need of percpu memory for bpf
>> program. With this change, percpu memory consumption after boot can be reduced
>> signicantly.
>>
>>    [1] https://lore.kernel.org/lkml/20231109154934.4saimljtqx625l3v@box.shutemov.name/
>>
>> Fixes: 41a5db8d8161 ("bpf: Add support for non-fix-size percpu mem allocation")
>> Reported-and-tested-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
>> ---
>>   include/linux/bpf.h   |  2 +-
>>   kernel/bpf/core.c     |  8 +++-----
>>   kernel/bpf/verifier.c | 20 ++++++++++++++++++--
>>   3 files changed, 22 insertions(+), 8 deletions(-)
>>
>> Changelog:
>>    v2 -> v3:
>>      - Use dedicated mutex lock (bpf_percpu_ma_lock)
>>    v1 -> v2:
>>      - Add proper Reported-and-tested-by tag.
>>      - Do a check of !bpf_global_percpu_ma_set before acquiring verifier_lock.
>>
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> index 35bff17396c0..6762dac3ef76 100644
>> --- a/include/linux/bpf.h
>> +++ b/include/linux/bpf.h
>> @@ -56,7 +56,7 @@ extern struct idr btf_idr;
>>   extern spinlock_t btf_idr_lock;
>>   extern struct kobject *btf_kobj;
>>   extern struct bpf_mem_alloc bpf_global_ma, bpf_global_percpu_ma;
>> -extern bool bpf_global_ma_set, bpf_global_percpu_ma_set;
>> +extern bool bpf_global_ma_set;
>>   
>>   typedef u64 (*bpf_callback_t)(u64, u64, u64, u64, u64);
>>   typedef int (*bpf_iter_init_seq_priv_t)(void *private_data,
>> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
>> index 08626b519ce2..cd3afe57ece3 100644
>> --- a/kernel/bpf/core.c
>> +++ b/kernel/bpf/core.c
>> @@ -64,8 +64,8 @@
>>   #define OFF	insn->off
>>   #define IMM	insn->imm
>>   
>> -struct bpf_mem_alloc bpf_global_ma, bpf_global_percpu_ma;
>> -bool bpf_global_ma_set, bpf_global_percpu_ma_set;
>> +struct bpf_mem_alloc bpf_global_ma;
>> +bool bpf_global_ma_set;
>>   
>>   /* No hurry in this branch
>>    *
>> @@ -2934,9 +2934,7 @@ static int __init bpf_global_ma_init(void)
>>   
>>   	ret = bpf_mem_alloc_init(&bpf_global_ma, 0, false);
>>   	bpf_global_ma_set = !ret;
>> -	ret = bpf_mem_alloc_init(&bpf_global_percpu_ma, 0, true);
>> -	bpf_global_percpu_ma_set = !ret;
>> -	return !bpf_global_ma_set || !bpf_global_percpu_ma_set;
>> +	return ret;
>>   }
>>   late_initcall(bpf_global_ma_init);
>>   #endif
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index a2267d5ed14e..6da370a047fe 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -26,6 +26,7 @@
>>   #include <linux/poison.h>
>>   #include <linux/module.h>
>>   #include <linux/cpumask.h>
>> +#include <linux/bpf_mem_alloc.h>
>>   #include <net/xdp.h>
>>   
>>   #include "disasm.h"
>> @@ -41,6 +42,9 @@ static const struct bpf_verifier_ops * const bpf_verifier_ops[] = {
>>   #undef BPF_LINK_TYPE
>>   };
>>   
>> +struct bpf_mem_alloc bpf_global_percpu_ma;
>> +static bool bpf_global_percpu_ma_set;
>> +
>>   /* bpf_check() is a static code analyzer that walks eBPF program
>>    * instruction by instruction and updates register/stack state.
>>    * All paths of conditional branches are analyzed until 'bpf_exit' insn.
>> @@ -336,6 +340,7 @@ struct bpf_kfunc_call_arg_meta {
>>   struct btf *btf_vmlinux;
>>   
>>   static DEFINE_MUTEX(bpf_verifier_lock);
>> +static DEFINE_MUTEX(bpf_percpu_ma_lock);
>>   
>>   static const struct bpf_line_info *
>>   find_linfo(const struct bpf_verifier_env *env, u32 insn_off)
>> @@ -12091,8 +12096,19 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>>   				if (meta.func_id == special_kfunc_list[KF_bpf_obj_new_impl] && !bpf_global_ma_set)
>>   					return -ENOMEM;
>>   
>> -				if (meta.func_id == special_kfunc_list[KF_bpf_percpu_obj_new_impl] && !bpf_global_percpu_ma_set)
>> -					return -ENOMEM;
>> +				if (meta.func_id == special_kfunc_list[KF_bpf_percpu_obj_new_impl]) {
>> +					if (!bpf_global_percpu_ma_set) {
>> +						mutex_lock(&bpf_percpu_ma_lock);
>> +						if (!bpf_global_percpu_ma_set) {
>> +							err = bpf_mem_alloc_init(&bpf_global_percpu_ma, 0, true);
>> +							if (!err)
>> +								bpf_global_percpu_ma_set = true;
>> +						}
> A dumb question here: do we need some memory barrier to guarantee the
> memory order between bpf_global_percpu_ma_set and bpf_global_percpu_ma ?

We should be fine. There is a control dependence on '!err' for
'bpf_global_percpu_ma_set = true'.

>> +						mutex_unlock(&bpf_percpu_ma_lock);
>> +						if (err)
>> +							return err;
>> +					}
>> +				}
>>   
>>   				if (((u64)(u32)meta.arg_constant.value) != meta.arg_constant.value) {
>>   					verbose(env, "local type ID argument must be in range [0, U32_MAX]\n");