From: "Jan Beulich" <JBeulich@suse.com>
To: Dario Faggioli <dario.faggioli@citrix.com>,
George Dunlap <george.dunlap@citrix.com>
Cc: George Dunlap <george.dunlap@eu.citrix.com>,
Andrew Cooper <andrew.cooper3@citrix.com>,
Wei Liu <wei.liu2@citrix.com>,
Ian Jackson <ian.jackson@eu.citrix.com>,
xen-devel@lists.xenproject.org
Subject: Re: [PATCH 03/15] xen/tools: tracing: several improvements on IRQs tracing
Date: Thu, 08 Jun 2017 09:34:17 -0600 [thread overview]
Message-ID: <59398A990200007800160F31@prv-mh.provo.novell.com> (raw)
In-Reply-To: <173fc819-544d-2711-2546-f174e7df7fa9@citrix.com>
>>> On 08.06.17 at 16:53, <george.dunlap@citrix.com> wrote:
> On 07/06/17 16:58, Jan Beulich wrote:
>>>>> On 07.06.17 at 17:45, <dario.faggioli@citrix.com> wrote:
>>> On Wed, 2017-06-07 at 09:05 -0600, Jan Beulich wrote:
>>>>>>> On 01.06.17 at 19:33, <dario.faggioli@citrix.com> wrote:
>>>>> @@ -884,9 +919,13 @@ void do_IRQ(struct cpu_user_regs *regs)
>>>>> desc->rl_quantum_start = now;
>>>>> }
>>>>>
>>>>> - tsc_in = tb_init_done ? get_cycles() : 0;
>>>>> + if ( unlikely(tb_init_done) )
>>>>> + {
>>>>> + __trace_var(TRC_HW_IRQ_GUEST, 0, sizeof(irq), &irq);
>>>>> + tsc_in = get_cycles();
>>>>> + }
>>>>> __do_IRQ_guest(irq);
>>>>> - TRACE_3D(TRC_HW_IRQ_HANDLED, irq, tsc_in, get_cycles());
>>>>> + trace_irq_handled(irq, get_cycles() - tsc_in);
>>>>> goto out_no_end;
>>>>> }
>>>>
>>>> Before this change, the get_cycles() invocation was after
>>>> the tb_init_done check. Now you move it ahead of it (as
>>>> the function arguments are evaluated before executing the
>>>> function body) - are you sure all compiler versions will suitably
>>>> re-order this?
>>>>
>>>> Additionally I'm afraid this will trigger compiler warnings on at
>>>> least some compiler versions, as tsc_in is now possibly
>>>> uninitialized (and there's no clear way to disprove this for the
>>>> compiler, again because function arguments are being
>>>> evaluated before the function body is reached).
>>>>
>>> I understand and (now that I see it) agree with your remark on when
>>> get_cycles() is called. I'll reorganize things so that the patched
>>> behavior matches the existing one.
>>>
>>> OTOH, I'm not sure I see the "potentially uninitialized" issue for
>>> tsc_in, but since I'm reworking the code, I'll keep that in mind too.
>>
>> You initialize tsc_in only when tb_init_done is set, but you use
>> it without that conditional. And even if you used it under that
>> same conditional, older compiler versions aren't able to track
>> that fact (same conditional) and raise a warning anyway.
>
> This patch changes thing to initialize tsc_in on declaration (at the top
> of the function).
Oh, it's only now that I actually see an initializer is being added.
I'm sorry for the noise then.
> If we want to keep the compiler "uninitialized variable" analysis
> around, that would have to go away and we'd want to do something like
> was there before. (I'm ambivalent about it, but I know Jan likes it.)
No, I'm fine with the change. I was just being blind.
Jan
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next prev parent reply other threads:[~2017-06-08 15:34 UTC|newest]
Thread overview: 68+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-01 17:33 [PATCH 00/15] xen/tools: add tracing to various Xen subsystems Dario Faggioli
2017-06-01 17:33 ` [PATCH 01/15] xen: in do_softirq() sample smp_processor_id() once and for all Dario Faggioli
2017-06-07 14:38 ` Jan Beulich
2017-06-08 14:12 ` George Dunlap
2017-06-08 14:20 ` George Dunlap
2017-06-08 14:42 ` Jan Beulich
2017-06-01 17:33 ` [PATCH 02/15] xen: tracing: avoid checking tb_init_done multiple times Dario Faggioli
2017-06-01 17:53 ` Andrew Cooper
2017-06-01 23:08 ` Dario Faggioli
2017-06-07 14:46 ` Jan Beulich
2017-06-07 15:55 ` Dario Faggioli
2017-06-07 16:06 ` Jan Beulich
2017-06-08 14:34 ` George Dunlap
2017-06-08 14:37 ` George Dunlap
2017-06-01 17:33 ` [PATCH 03/15] xen/tools: tracing: several improvements on IRQs tracing Dario Faggioli
2017-06-01 18:02 ` Andrew Cooper
2017-06-01 23:12 ` Dario Faggioli
2017-06-07 15:05 ` Jan Beulich
2017-06-07 15:45 ` Dario Faggioli
2017-06-07 15:58 ` Jan Beulich
2017-06-08 14:53 ` George Dunlap
2017-06-08 15:34 ` Jan Beulich [this message]
2017-06-08 14:59 ` George Dunlap
2017-06-01 17:34 ` [PATCH 04/15] tools: xenalyze: fix dumping of PM_IDLE events Dario Faggioli
2017-06-08 15:06 ` George Dunlap
2017-06-01 17:34 ` [PATCH 05/15] xen: make it possible to disable tracing in Kconfig Dario Faggioli
2017-06-01 18:43 ` Andrew Cooper
2017-06-07 11:01 ` Julien Grall
2017-06-07 15:14 ` Jan Beulich
2017-06-08 15:16 ` George Dunlap
2017-06-08 15:35 ` Jan Beulich
2017-06-08 15:37 ` George Dunlap
2017-06-08 15:44 ` Jan Beulich
2017-06-08 15:17 ` George Dunlap
2017-06-01 17:34 ` [PATCH 06/15] xen: trace IRQ enabling/disabling Dario Faggioli
2017-06-01 19:08 ` Andrew Cooper
2017-06-01 23:42 ` Dario Faggioli
2017-06-08 15:51 ` George Dunlap
2017-06-08 16:05 ` Jan Beulich
2017-06-07 11:16 ` Julien Grall
2017-06-07 15:22 ` Dario Faggioli
2017-06-09 10:51 ` Julien Grall
2017-06-09 10:53 ` Julien Grall
2017-06-09 10:55 ` George Dunlap
2017-06-09 11:00 ` Julien Grall
2017-06-08 16:01 ` George Dunlap
2017-06-08 16:11 ` Dario Faggioli
2017-06-09 10:41 ` Jan Beulich
2017-06-01 17:34 ` [PATCH 07/15] tools: tracing: handle IRQs on/off events in xentrace and xenalyze Dario Faggioli
2017-06-13 15:58 ` George Dunlap
2017-06-01 17:34 ` [PATCH 08/15] xen: trace RCU behavior Dario Faggioli
2017-06-09 10:48 ` Jan Beulich
2017-06-13 16:05 ` George Dunlap
2017-06-01 17:34 ` [PATCH 09/15] tools: tracing: handle RCU events in xentrace and xenalyze Dario Faggioli
2017-06-13 16:12 ` George Dunlap
2017-06-01 17:34 ` [PATCH 10/15] xen: trace softirqs Dario Faggioli
2017-06-09 10:51 ` Jan Beulich
2017-06-01 17:34 ` [PATCH 11/15] tools: tracing: handle RCU events in xentrace and xenalyze Dario Faggioli
2017-06-01 17:35 ` [PATCH 12/15] xen: trace tasklets Dario Faggioli
2017-06-09 10:59 ` Jan Beulich
2017-06-09 11:17 ` Dario Faggioli
2017-06-09 11:29 ` Jan Beulich
2017-06-01 17:35 ` [PATCH 13/15] tools: tracing: handle tasklets events in xentrace and xenalyze Dario Faggioli
2017-06-01 17:35 ` [PATCH 14/15] xen: trace timers Dario Faggioli
2017-06-01 17:35 ` [PATCH 15/15] tools: tracing: handle timers events in xentrace and xenalyze Dario Faggioli
2017-06-07 14:13 ` [PATCH 00/15] xen/tools: add tracing to various Xen subsystems Konrad Rzeszutek Wilk
2017-06-08 16:45 ` Dario Faggioli
2017-06-13 16:34 ` George Dunlap
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