linux-kernel - Re: [PATCH 5/8] trace_uprobe: Support SDT markers having reference count (semaphore)

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Date:   Wed, 14 Mar 2018 20:42:24 +0530
From:   Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     oleg@...hat.com, peterz@...radead.org, srikar@...ux.vnet.ibm.com,
        acme@...nel.org, ananth@...ux.vnet.ibm.com,
        akpm@...ux-foundation.org, alexander.shishkin@...ux.intel.com,
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        tmricht@...ux.vnet.ibm.com, willy@...radead.org,
        yao.jin@...ux.intel.com, fengguang.wu@...el.com,
        Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com>
Subject: Re: [PATCH 5/8] trace_uprobe: Support SDT markers having reference
 count (semaphore)

Hi Masami,

On 03/14/2018 07:18 PM, Masami Hiramatsu wrote:
> Hi Ravi,
>
> On Tue, 13 Mar 2018 18:26:00 +0530
> Ravi Bangoria <ravi.bangoria@...ux.vnet.ibm.com> wrote:
>
>> Userspace Statically Defined Tracepoints[1] are dtrace style markers
>> inside userspace applications. These markers are added by developer at
>> important places in the code. Each marker source expands to a single
>> nop instruction in the compiled code but there may be additional
>> overhead for computing the marker arguments which expands to couple of
>> instructions. In case the overhead is more, execution of it can be
>> ommited by runtime if() condition when no one is tracing on the marker:
>>
>>     if (reference_counter > 0) {
>>         Execute marker instructions;
>>     }
>>
>> Default value of reference counter is 0. Tracer has to increment the
>> reference counter before tracing on a marker and decrement it when
>> done with the tracing.
>>
>> Implement the reference counter logic in trace_uprobe, leaving core
>> uprobe infrastructure as is, except one new callback from uprobe_mmap()
>> to trace_uprobe.
>>
>> trace_uprobe definition with reference counter will now be:
>>
>>   <path>:<offset>[(ref_ctr_offset)]
> Would you mean 
> <path>:<offset>(<ref_ctr_offset>)
> ?
>
> or use "[]" for delimiter?

[] indicates optional field.

> Since,
>
>> @@ -454,6 +458,26 @@ static int create_trace_uprobe(int argc, char **argv)
>>  		goto fail_address_parse;
>>  	}
>>  
>> +	/* Parse reference counter offset if specified. */
>> +	rctr = strchr(arg, '(');
> This seems you choose "()" for delimiter.

Correct.

>> +	if (rctr) {
>> +		rctr_end = strchr(arg, ')');
> 		rctr_end = strchr(rctr, ')');
>
> ? since we are sure rctr != NULL.

Yes. we can use rctr instead of arg.

>> +		if (rctr > rctr_end || *(rctr_end + 1) != 0) {
>> +			ret = -EINVAL;
>> +			pr_info("Invalid reference counter offset.\n");
>> +			goto fail_address_parse;
>> +		}
>
> Also
>
>> +
>> +		*rctr++ = 0;
>> +		*rctr_end = 0;
> Please consider to use '\0' for nul;

Sure. Will change it.

Thanks for the review :)
Ravi