linux-kernel - Re: [PATCH 2/3] perf/core: Set data->sample_flags in perf_prepare

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Message-ID: <Y76xng1U6UYpIGaW@hirez.programming.kicks-ass.net>
Date:   Wed, 11 Jan 2023 13:54:54 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Ravi Bangoria <ravi.bangoria@....com>, bpf@...r.kernel.org
Subject: Re: [PATCH 2/3] perf/core: Set data->sample_flags in
 perf_prepare_sample()

On Tue, Jan 10, 2023 at 12:06:00PM -0800, Namhyung Kim wrote:

> Another example, but in this case it's real, is ADDR.  We cannot update
> the data->addr just because filtered_sample_type has PHYS_ADDR or
> DATA_PAGE_SIZE as it'd lose the original value.

Hmm, how about something like so?

/*
 * if (flags & s) flags |= d; // without branches
 */
static __always_inline unsigned long
__cond_set(unsigned long flags, unsigned long s, unsigned long d)
{
	return flags | (d * !!(flags & s));
}

Then:

	fst = sample_type;
	fst = __cond_set(fst, PERF_SAMPLE_CODE_PAGE_SIZE, PERF_SAMPLE_IP);
	fst = __cond_set(fst, PERF_SAMPLE_DATA_PAGE_SIZE |
			      PERF_SAMPLE_PHYS_ADDR,	  PERF_SAMPLE_ADDR);
	fst = __cond_set(fst, PERF_SAMPLE_STACK_USER,     PERF_SAMPLE_REGS_USER);
	fst &= ~data->sample_flags;

This way we express the implicit conditions by setting the required
sample data flags, then we mask those we already have set.

After the above something like:

	if (fst & PERF_SAMPLE_ADDR) {
		data->addr = 0;
		data->sample_flags |= PERF_SAMPLE_ADDR;
	}

	if (fst & PERF_SAMPLE_PHYS_ADDR) {
		data->phys_addr = perf_virt_to_phys(data->addr);
		data->sample_flags |= PERF_SAMPLE_PHYS_ADDR;
	}

	if (fst & PERF_SAMPLE_DATA_PAGE_SIZE) {
		data->data_page_size = perf_get_page_size(data->addr);
		data->sample_flags |= PERF_SAMPLE_DATA_PAGE_SIZE;
	}

And maybe something like:

#define __IF_SAMPLE_DATA(f_)		({		\
	bool __f = fst & PERF_SAMPLE_##f_;		\
	if (__f) data->sample_flags |= PERF_SAMPLE_##f_;\
	__f;				})

#define IF_SAMPLE_DATA(f_) if (__IF_SAMPLE_DATA(f_))

Then we can write:

	IF_SAMPLE_DATA(ADDR)
		data->addr = 0;

	IF_SAMPLE_DATA(PHYS_ADDR)
		data->phys_addr = perf_virt_to_phys(data->addr);

	IF_SAMPLE_DATA(DATA_PAGE_SIZE)
		data->data_page_size = perf_get_page_size(data->addr);

But I didn't check code-gen for this last suggestion.