linux-kernel - Re: [[PATCH v6 02/10] perf: Add a flags parameter to pmu txn interfaces

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Date:	Fri, 04 Sep 2015 20:07:29 +1000
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	linux-s390@...r.kernel.org, sparclinux@...r.kernel.org
Subject: Re: [[PATCH v6 02/10] perf: Add a flags parameter to pmu txn
 interfaces

On Thu, 2015-09-03 at 20:07 -0700, Sukadev Bhattiprolu wrote:
> Currently, the PMU interface allows reading only one counter at a time.
> But some PMUs like the 24x7 counters in Power, support reading several
> counters at once. To leveage this functionality, extend the transaction
> interface to support a "transaction type".
> 
> The first type, PERF_PMU_TXN_ADD, refers to the existing transactions,
> i.e. used to _schedule_ all the events on the PMU as a group. A second
> transaction type, PERF_PMU_TXN_READ, will be used in a follow-on patch,
> by the 24x7 counters to read several counters at once.
> 
> Extend the transaction interfaces to the PMU to accept a 'txn_flags'
> parameter and use this parameter to ignore any transactions that are
> not of type PERF_PMU_TXN_ADD.
> 
> Thanks to Peter Zijlstra for his input.
> 
> Signed-off-by: Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
> 
> ---
>  arch/powerpc/perf/core-book3s.c  |   30 +++++++++++++++++++++++++++++-

These powerpc changes look OK to me.

So for those you can have an:

Acked-by: Michael Ellerman <mpe@...erman.id.au>


Having said that, we do end up repeating a lot of boiler plate for each arch,
which is a pity. eg:

> +static void power_pmu_start_txn(struct pmu *pmu, unsigned int txn_flags)
>  {
>  	struct cpu_hw_events *cpuhw = this_cpu_ptr(&cpu_hw_events);
>
> +	WARN_ON_ONCE(cpuhw->txn_flags);		/* txn already in flight */
> +
> +	cpuhw->txn_flags = txn_flags;
> +	if (txn_flags & ~PERF_PMU_TXN_ADD)
> +		return;
> +

And so on.

But I can't think of an easy way to avoid that, so it's not a blocker, but
maybe someone can think of a nice solution to avoid it?

cheers



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