linux-kernel - Re: [PATCH v2 2/2] powercap/rapl: reduce ipi calls

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Message-ID: <alpine.DEB.2.11.1601132257550.3575@nanos>
Date:	Wed, 13 Jan 2016 23:02:33 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
cc:	Borislav Petkov <bp@...en8.de>,
	Jacob Pan <jacob.jun.pan@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux PM <linux-pm@...r.kernel.org>,
	Rafael Wysocki <rafael.j.wysocki@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	X86 Kernel <x86@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 2/2] powercap/rapl: reduce ipi calls

On Wed, 13 Jan 2016, Srinivas Pandruvada wrote:
> On Wed, 2016-01-13 at 22:26 +0100, Borislav Petkov wrote:
> > 	rmwmsrl_safe_on_cpu(policy->cpu, MSR_IA32_PERF_CTL,
> > 			    INTEL_PERF_CTL_MASK,
> > 			    (u32)sfi_cpufreq_array[next_perf_state].ctr
> > l_val & INTEL_PERF_CTL_MASK);
> > 
> > Yikes!
> > 
> > So yes, it can work but it is ugly, hard to parse and use, not
> > generic
> > enough, etc, etc.
> > 
> > So thanks, but no thanks.
> > 
> I agree, in some cases it will not make much sense to use read-
> modify_write calls, the user may decide whether it makes sense or not.
> But such interface is not new to Linux kernel:
> 
> regmap_update_bits(), which is referenced for 346 times.
> 
> Are you saying that any such calls are not useful?

There are certainly cases when such calls are useful. And those cases are when
we have a sufficiently big occurence of similar code which is sufficiently
complex to justify the library code and the export.

Thanks,

	tglx