linux-kernel - Re: [PATCH v2] uprobes: make trace_uprobe->nhit counter a per-CPU one

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Message-Id: <20240813223014.1a5093ede1a5046aaedea34a@kernel.org>
Date: Tue, 13 Aug 2024 22:30:14 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Andrii Nakryiko <andrii@...nel.org>
Cc: linux-trace-kernel@...r.kernel.org, rostedt@...dmis.org,
 peterz@...radead.org, oleg@...hat.com, bpf@...r.kernel.org,
 linux-kernel@...r.kernel.org, jolsa@...nel.org
Subject: Re: [PATCH v2] uprobes: make trace_uprobe->nhit counter a per-CPU
 one

On Fri,  9 Aug 2024 12:23:57 -0700
Andrii Nakryiko <andrii@...nel.org> wrote:

> trace_uprobe->nhit counter is not incremented atomically, so its value
> is questionable in when uprobe is hit on multiple CPUs simultaneously.
> 
> Also, doing this shared counter increment across many CPUs causes heavy
> cache line bouncing, limiting uprobe/uretprobe performance scaling with
> number of CPUs.
> 
> Solve both problems by making this a per-CPU counter.
> 

This looks good to me. I would like to pick this to linux-trace/probes/for-next.

> @@ -62,7 +63,7 @@ struct trace_uprobe {
>  	struct uprobe			*uprobe;

BTW, what is this change? I couldn't cleanly apply this to the v6.11-rc3.
Which tree would you working on? (I missed something?)

Thanks,

>  	unsigned long			offset;
>  	unsigned long			ref_ctr_offset;
> -	unsigned long			nhit;
> +	unsigned long __percpu		*nhits;
>  	struct trace_probe		tp;
>  };
>  
> @@ -337,6 +338,12 @@ alloc_trace_uprobe(const char *group, const char *event, int nargs, bool is_ret)
>  	if (!tu)
>  		return ERR_PTR(-ENOMEM);
>  
> +	tu->nhits = alloc_percpu(unsigned long);
> +	if (!tu->nhits) {
> +		ret = -ENOMEM;
> +		goto error;
> +	}
> +
>  	ret = trace_probe_init(&tu->tp, event, group, true, nargs);
>  	if (ret < 0)
>  		goto error;
> @@ -349,6 +356,7 @@ alloc_trace_uprobe(const char *group, const char *event, int nargs, bool is_ret)
>  	return tu;
>  
>  error:
> +	free_percpu(tu->nhits);
>  	kfree(tu);
>  
>  	return ERR_PTR(ret);
> @@ -362,6 +370,7 @@ static void free_trace_uprobe(struct trace_uprobe *tu)
>  	path_put(&tu->path);
>  	trace_probe_cleanup(&tu->tp);
>  	kfree(tu->filename);
> +	free_percpu(tu->nhits);
>  	kfree(tu);
>  }
>  
> @@ -815,13 +824,21 @@ static int probes_profile_seq_show(struct seq_file *m, void *v)
>  {
>  	struct dyn_event *ev = v;
>  	struct trace_uprobe *tu;
> +	unsigned long nhits;
> +	int cpu;
>  
>  	if (!is_trace_uprobe(ev))
>  		return 0;
>  
>  	tu = to_trace_uprobe(ev);
> +
> +	nhits = 0;
> +	for_each_possible_cpu(cpu) {
> +		nhits += READ_ONCE(*per_cpu_ptr(tu->nhits, cpu));
> +	}
> +
>  	seq_printf(m, "  %s %-44s %15lu\n", tu->filename,
> -			trace_probe_name(&tu->tp), tu->nhit);
> +		   trace_probe_name(&tu->tp), nhits);
>  	return 0;
>  }
>  
> @@ -1507,7 +1524,8 @@ static int uprobe_dispatcher(struct uprobe_consumer *con, struct pt_regs *regs)
>  	int ret = 0;
>  
>  	tu = container_of(con, struct trace_uprobe, consumer);
> -	tu->nhit++;
> +
> +	this_cpu_inc(*tu->nhits);
>  
>  	udd.tu = tu;
>  	udd.bp_addr = instruction_pointer(regs);
> -- 
> 2.43.5
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>