From: Peter Zijlstra <peterz@infradead.org>
To: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
Cc: linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org,
joro@8bytes.org, bp@alien8.de, mingo@redhat.com
Subject: Re: [PATCH v8 3/9] perf/amd/iommu: Misc fix up perf_iommu_read
Date: Mon, 23 Jan 2017 13:33:30 +0100 [thread overview]
Message-ID: <20170123123330.GY6485@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <1484551416-5440-4-git-send-email-Suravee.Suthikulpanit@amd.com>
On Mon, Jan 16, 2017 at 01:23:30AM -0600, Suravee Suthikulpanit wrote:
> static void perf_iommu_read(struct perf_event *event)
> {
> - u64 count = 0ULL;
> - u64 prev_raw_count = 0ULL;
> - u64 delta = 0ULL;
> + u64 count, prev;
> + s64 delta;
I did send that email where I told I was mistaken with that suggestion,
right? Since the counter always increments (it does, right), a negative
delta does not make sense.
> struct hw_perf_event *hwc = &event->hw;
>
> amd_iommu_pc_get_set_reg_val(_GET_DEVID(event),
> @@ -330,18 +329,20 @@ static void perf_iommu_read(struct perf_event *event)
> IOMMU_PC_COUNTER_REG, &count, false);
>
> /* IOMMU pc counter register is only 48 bits */
> - count &= 0xFFFFFFFFFFFFULL;
> + count &= GENMASK_ULL(48, 0);
Why do you need that at all? If the counter is only 48 bits, what does
the hardware do with the upper bits?
>
> - prev_raw_count = local64_read(&hwc->prev_count);
> - if (local64_cmpxchg(&hwc->prev_count, prev_raw_count,
> - count) != prev_raw_count)
> - return;
> + prev = local64_read(&hwc->prev_count);
I'm still not convinced you can do away with that cmpxchg.
>
> - /* Handling 48-bit counter overflowing */
> - delta = (count << COUNTER_SHIFT) - (prev_raw_count << COUNTER_SHIFT);
> + /*
> + * Since we do not enable counter overflow interrupts,
> + * we do not have to worry about prev_count changing on us.
> + */
> + local64_set(&hwc->prev_count, count);
> +
> + /* Handle 48-bit counter overflow */
> + delta = (count << COUNTER_SHIFT) - (prev << COUNTER_SHIFT);
> delta >>= COUNTER_SHIFT;
> local64_add(delta, &event->count);
> -
> }
>
> static void perf_iommu_stop(struct perf_event *event, int flags)
> --
> 1.8.3.1
>
next prev parent reply other threads:[~2017-01-23 12:33 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-16 7:23 [PATCH v8 0/9] perf/amd/iommu: Enable multi-IOMMU support Suravee Suthikulpanit
2017-01-16 7:23 ` [PATCH v8 1/9] perf/amd/iommu: Declare pr_fmt and remove unnecessary pr_debug Suravee Suthikulpanit
2017-01-16 7:23 ` [PATCH v8 2/9] perf/amd/iommu: Clean up perf_iommu_enable_event Suravee Suthikulpanit
2017-01-18 18:20 ` Borislav Petkov
2017-01-16 7:23 ` [PATCH v8 3/9] perf/amd/iommu: Misc fix up perf_iommu_read Suravee Suthikulpanit
2017-01-19 10:01 ` Borislav Petkov
2017-01-23 12:33 ` Peter Zijlstra [this message]
2017-02-07 4:50 ` Suravee Suthikulpanit
2017-01-16 7:23 ` [PATCH v8 4/9] iommu/amd: Introduce amd_iommu_get_num_iommus() Suravee Suthikulpanit
2017-01-19 18:41 ` Borislav Petkov
2017-01-16 7:23 ` [PATCH v8 5/9] perf/amd/iommu: Modify functions to query max banks and counters Suravee Suthikulpanit
2017-01-22 19:53 ` Borislav Petkov
2017-01-16 7:23 ` [PATCH v8 6/9] perf/amd/iommu: Modify amd_iommu_pc_get_set_reg_val() API to allow specifying IOMMU index Suravee Suthikulpanit
2017-01-22 19:53 ` Borislav Petkov
2017-01-16 7:23 ` [PATCH v8 7/9] perf/amd/iommu: Check return value when set and get counter value Suravee Suthikulpanit
2017-01-22 19:53 ` Borislav Petkov
2017-01-23 12:31 ` Peter Zijlstra
2017-01-16 7:23 ` [PATCH v8 8/9] perf/amd/iommu: Fix sysfs perf attribute groups Suravee Suthikulpanit
2017-01-22 19:54 ` Borislav Petkov
2017-01-16 7:23 ` [PATCH v8 9/9] perf/amd/iommu: Enable support for multiple IOMMUs Suravee Suthikulpanit
2017-01-22 19:55 ` Borislav Petkov
2017-02-07 1:42 ` Suravee Suthikulpanit
2017-01-25 9:46 ` Peter Zijlstra
2017-01-25 9:55 ` Borislav Petkov
2017-02-07 1:58 ` Suravee Suthikulpanit
2017-02-07 1:57 ` Suravee Suthikulpanit
2017-02-14 12:31 ` Peter Zijlstra
2017-02-23 17:43 ` Suravee Suthikulpanit
2017-02-23 18:11 ` Peter Zijlstra
2017-02-23 18:20 ` Suravee Suthikulpanit
2017-01-17 15:36 ` [PATCH v8 0/9] perf/amd/iommu: Enable multi-IOMMU support Joerg Roedel
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