ring_buffer.c

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/*
 * Performance events ring-buffer code:
 *
 *  Copyright (C) 2008 Thomas Gleixner <[email protected]>
 *  Copyright (C) 2008-2011 Red Hat, Inc., Ingo Molnar
 *  Copyright (C) 2008-2011 Red Hat, Inc., Peter Zijlstra <[email protected]>
 *  Copyright  ©  2009 Paul Mackerras, IBM Corp. <[email protected]>
 *
 * For licensing details see kernel-base/COPYING
 */

#include <linux/perf_event.h>
#include <linux/vmalloc.h>
#include <linux/slab.h>

#include "internal.h"

static bool perf_output_space(struct ring_buffer *rb, unsigned long tail,
                  unsigned long offset, unsigned long head)
{
    unsigned long mask;

    if (!rb->writable)
        return true;

    mask = perf_data_size(rb) - 1;

    offset = (offset - tail) & mask;
    head   = (head   - tail) & mask;

    if ((int)(head - offset) < 0)
        return false;

    return true;
}

static void perf_output_wakeup(struct perf_output_handle *handle)
{
    atomic_set(&handle->rb->poll, POLL_IN);

    handle->event->pending_wakeup = 1;
    irq_work_queue(&handle->event->pending);
}

/*
 * We need to ensure a later event_id doesn't publish a head when a former
 * event isn't done writing. However since we need to deal with NMIs we
 * cannot fully serialize things.
 *
 * We only publish the head (and generate a wakeup) when the outer-most
 * event completes.
 */
static void perf_output_get_handle(struct perf_output_handle *handle)
{
    struct ring_buffer *rb = handle->rb;

    preempt_disable();
    local_inc(&rb->nest);
    handle->wakeup = local_read(&rb->wakeup);
}

static void perf_output_put_handle(struct perf_output_handle *handle)
{
    struct ring_buffer *rb = handle->rb;
    unsigned long head;

again:
    head = local_read(&rb->head);

    /*
     * IRQ/NMI can happen here, which means we can miss a head update.
     */

    if (!local_dec_and_test(&rb->nest))
        goto out;

    /*
     * Publish the known good head. Rely on the full barrier implied
     * by atomic_dec_and_test() order the rb->head read and this
     * write.
     */
    rb->user_page->data_head = head;

    /*
     * Now check if we missed an update, rely on the (compiler)
     * barrier in atomic_dec_and_test() to re-read rb->head.
     */
    if (unlikely(head != local_read(&rb->head))) {
        local_inc(&rb->nest);
        goto again;
    }

    if (handle->wakeup != local_read(&rb->wakeup))
        perf_output_wakeup(handle);

out:
    preempt_enable();
}

int perf_output_begin(struct perf_output_handle *handle,
              struct perf_event *event, unsigned int size)
{
    struct ring_buffer *rb;
    unsigned long tail, offset, head;
    int have_lost;
    struct perf_sample_data sample_data;
    struct {
        struct perf_event_header header;
        u64          id;
        u64          lost;
    } lost_event;

    rcu_read_lock();
    /*
     * For inherited events we send all the output towards the parent.
     */
    if (event->parent)
        event = event->parent;

    rb = rcu_dereference(event->rb);
    if (!rb)
        goto out;

    handle->rb  = rb;
    handle->event   = event;

    if (!rb->nr_pages)
        goto out;

    have_lost = local_read(&rb->lost);
    if (have_lost) {
        lost_event.header.size = sizeof(lost_event);
        perf_event_header__init_id(&lost_event.header, &sample_data,
                       event);
        size += lost_event.header.size;
    }

    perf_output_get_handle(handle);

    do {
        /*
         * Userspace could choose to issue a mb() before updating the
         * tail pointer. So that all reads will be completed before the
         * write is issued.
         */
        tail = ACCESS_ONCE(rb->user_page->data_tail);
        smp_rmb();
        offset = head = local_read(&rb->head);
        head += size;
        if (unlikely(!perf_output_space(rb, tail, offset, head)))
            goto fail;
    } while (local_cmpxchg(&rb->head, offset, head) != offset);

    if (head - local_read(&rb->wakeup) > rb->watermark)
        local_add(rb->watermark, &rb->wakeup);

    handle->page = offset >> (PAGE_SHIFT + page_order(rb));
    handle->page &= rb->nr_pages - 1;
    handle->size = offset & ((PAGE_SIZE << page_order(rb)) - 1);
    handle->addr = rb->data_pages[handle->page];
    handle->addr += handle->size;
    handle->size = (PAGE_SIZE << page_order(rb)) - handle->size;

    if (have_lost) {
        lost_event.header.type = PERF_RECORD_LOST;
        lost_event.header.misc = 0;
        lost_event.id          = event->id;
        lost_event.lost        = local_xchg(&rb->lost, 0);

        perf_output_put(handle, lost_event);
        perf_event__output_id_sample(event, handle, &sample_data);
    }

    return 0;

fail:
    local_inc(&rb->lost);
    perf_output_put_handle(handle);
out:
    rcu_read_unlock();

    return -ENOSPC;
}

unsigned int perf_output_copy(struct perf_output_handle *handle,
              const void *buf, unsigned int len)
{
    return __output_copy(handle, buf, len);
}

unsigned int perf_output_skip(struct perf_output_handle *handle,
                  unsigned int len)
{
    return __output_skip(handle, NULL, len);
}

void perf_output_end(struct perf_output_handle *handle)
{
    perf_output_put_handle(handle);
    rcu_read_unlock();
}

static void
ring_buffer_init(struct ring_buffer *rb, long watermark, int flags)
{
    long max_size = perf_data_size(rb);

    if (watermark)
        rb->watermark = min(max_size, watermark);

    if (!rb->watermark)
        rb->watermark = max_size / 2;

    if (flags & RING_BUFFER_WRITABLE)
        rb->writable = 1;

    atomic_set(&rb->refcount, 1);

    INIT_LIST_HEAD(&rb->event_list);
    spin_lock_init(&rb->event_lock);
}

#ifndef CONFIG_PERF_USE_VMALLOC

/*
 * Back perf_mmap() with regular GFP_KERNEL-0 pages.
 */

struct page *
perf_mmap_to_page(struct ring_buffer *rb, unsigned long pgoff)
{
    if (pgoff > rb->nr_pages)
        return NULL;

    if (pgoff == 0)
        return virt_to_page(rb->user_page);

    return virt_to_page(rb->data_pages[pgoff - 1]);
}

static void *perf_mmap_alloc_page(int cpu)
{
    struct page *page;
    int node;

    node = (cpu == -1) ? cpu : cpu_to_node(cpu);
    page = alloc_pages_node(node, GFP_KERNEL | __GFP_ZERO, 0);
    if (!page)
        return NULL;

    return page_address(page);
}

struct ring_buffer *rb_alloc(int nr_pages, long watermark, int cpu, int flags)
{
    struct ring_buffer *rb;
    unsigned long size;
    int i;

    size = sizeof(struct ring_buffer);
    size += nr_pages * sizeof(void *);

    rb = kzalloc(size, GFP_KERNEL);
    if (!rb)
        goto fail;

    rb->user_page = perf_mmap_alloc_page(cpu);
    if (!rb->user_page)
        goto fail_user_page;

    for (i = 0; i < nr_pages; i++) {
        rb->data_pages[i] = perf_mmap_alloc_page(cpu);
        if (!rb->data_pages[i])
            goto fail_data_pages;
    }

    rb->nr_pages = nr_pages;

    ring_buffer_init(rb, watermark, flags);

    return rb;

fail_data_pages:
    for (i--; i >= 0; i--)
        free_page((unsigned long)rb->data_pages[i]);

    free_page((unsigned long)rb->user_page);

fail_user_page:
    kfree(rb);

fail:
    return NULL;
}

static void perf_mmap_free_page(unsigned long addr)
{
    struct page *page = virt_to_page((void *)addr);

    page->mapping = NULL;
    __free_page(page);
}

void rb_free(struct ring_buffer *rb)
{
    int i;

    perf_mmap_free_page((unsigned long)rb->user_page);
    for (i = 0; i < rb->nr_pages; i++)
        perf_mmap_free_page((unsigned long)rb->data_pages[i]);
    kfree(rb);
}

#else

struct page *
perf_mmap_to_page(struct ring_buffer *rb, unsigned long pgoff)
{
    if (pgoff > (1UL << page_order(rb)))
        return NULL;

    return vmalloc_to_page((void *)rb->user_page + pgoff * PAGE_SIZE);
}

static void perf_mmap_unmark_page(void *addr)
{
    struct page *page = vmalloc_to_page(addr);

    page->mapping = NULL;
}

static void rb_free_work(struct work_struct *work)
{
    struct ring_buffer *rb;
    void *base;
    int i, nr;

    rb = container_of(work, struct ring_buffer, work);
    nr = 1 << page_order(rb);

    base = rb->user_page;
    for (i = 0; i < nr + 1; i++)
        perf_mmap_unmark_page(base + (i * PAGE_SIZE));

    vfree(base);
    kfree(rb);
}

void rb_free(struct ring_buffer *rb)
{
    schedule_work(&rb->work);
}

struct ring_buffer *rb_alloc(int nr_pages, long watermark, int cpu, int flags)
{
    struct ring_buffer *rb;
    unsigned long size;
    void *all_buf;

    size = sizeof(struct ring_buffer);
    size += sizeof(void *);

    rb = kzalloc(size, GFP_KERNEL);
    if (!rb)
        goto fail;

    INIT_WORK(&rb->work, rb_free_work);

    all_buf = vmalloc_user((nr_pages + 1) * PAGE_SIZE);
    if (!all_buf)
        goto fail_all_buf;

    rb->user_page = all_buf;
    rb->data_pages[0] = all_buf + PAGE_SIZE;
    rb->page_order = ilog2(nr_pages);
    rb->nr_pages = 1;

    ring_buffer_init(rb, watermark, flags);

    return rb;

fail_all_buf:
    kfree(rb);

fail:
    return NULL;
}

#endif