Re: [PATCH 1/2] uprobes: Optimize the return_instance related routines

["Liao, Chang" <liaochang1@huawei.com>]

在 2024/7/10 7:55, Andrii Nakryiko 写道:
> On Mon, Jul 8, 2024 at 6:00 PM Liao Chang <liaochang1@huawei.com> wrote:
>>
>> Reduce the runtime overhead for struct return_instance data managed by
>> uretprobe. This patch replaces the dynamic allocation with statically
>> allocated array, leverage two facts that are limited nesting depth of
>> uretprobe (max 64) and the function call style of return_instance usage
>> (create at entry, free at exit).
>>
>> This patch has been tested on Kunpeng916 (Hi1616), 4 NUMA nodes, 64
>> cores @ 2.4GHz. Redis benchmarks show a throughput gain by 2% for Redis
>> GET and SET commands:
>>
>> ------------------------------------------------------------------
>> Test case       | No uretprobes | uretprobes     | uretprobes
>>                 |               | (current)      | (optimized)
>> ==================================================================
>> Redis SET (RPS) | 47025         | 40619 (-13.6%) | 41529 (-11.6%)
>> ------------------------------------------------------------------
>> Redis GET (RPS) | 46715         | 41426 (-11.3%) | 42306 (-9.4%)
>> ------------------------------------------------------------------
>>
>> Signed-off-by: Liao Chang <liaochang1@huawei.com>
>> ---
>>  include/linux/uprobes.h |  10 ++-
>>  kernel/events/uprobes.c | 162 ++++++++++++++++++++++++----------------
>>  2 files changed, 105 insertions(+), 67 deletions(-)
>>
> 
> [...]
> 
>> +static void cleanup_return_instances(struct uprobe_task *utask, bool chained,
>> +                                    struct pt_regs *regs)
>> +{
>> +       struct return_frame *frame = &utask->frame;
>> +       struct return_instance *ri = frame->return_instance;
>> +       enum rp_check ctx = chained ? RP_CHECK_CHAIN_CALL : RP_CHECK_CALL;
>> +
>> +       while (ri && !arch_uretprobe_is_alive(ri, ctx, regs)) {
>> +               ri = next_ret_instance(frame, ri);
>> +               utask->depth--;
>> +       }
>> +       frame->return_instance = ri;
>> +}
>> +
>> +static struct return_instance *alloc_return_instance(struct uprobe_task *task)
>> +{
>> +       struct return_frame *frame = &task->frame;
>> +
>> +       if (!frame->vaddr) {
>> +               frame->vaddr = kcalloc(MAX_URETPROBE_DEPTH,
>> +                               sizeof(struct return_instance), GFP_KERNEL);
> 
> Are you just pre-allocating MAX_URETPROBE_DEPTH instances always?
> I.e., even if we need just one (because there is no recursion), you'd
> still waste memory for all 64 ones?

This is the truth. On my testing machines, each struct return_instance data
is 28 bytes, resulting in a total pre-allocated 1792 bytes when the first
instrumented function is hit.

> 
> That seems rather wasteful.
> 
> Have you considered using objpool for fast reuse across multiple CPUs?
> Check lib/objpool.c.

After studying how kretprobe uses objpool, I'm convinced it is a right solution for
managing return_instance in uretporbe. While I need some time to fully understand
the objpool code itself and run some benchmark to verify its performance.

Thanks for the suggestion.

> 
>> +               if (!frame->vaddr)
>> +                       return NULL;
>> +       }
>> +
>> +       if (!frame->return_instance) {
>> +               frame->return_instance = frame->vaddr;
>> +               return frame->return_instance;
>> +       }
>> +
>> +       return ++frame->return_instance;
>> +}
>> +
>> +static inline bool return_frame_empty(struct uprobe_task *task)
>> +{
>> +       return !task->frame.return_instance;
>>  }
>>
>>  /*
> 
> [...]

-- 
BR
Liao, Chang