On Mon, Jun 21, 2021 at 1:31 AM SeongJae Park <sj38.park@xxxxxxxxx> wrote:
>
> From: SeongJae Park <sjpark@xxxxxxxxx>
>
> This commit introduces a reference implementation of the address space
> specific low level primitives for the virtual address space, so that
> users of DAMON can easily monitor the data accesses on virtual address
> spaces of specific processes by simply configuring the implementation to
> be used by DAMON.
>
> The low level primitives for the fundamental access monitoring are
> defined in two parts:
>
> 1. Identification of the monitoring target address range for the address
> space.
> 2. Access check of specific address range in the target space.
>
> The reference implementation for the virtual address space does the
> works as below.
>
> PTE Accessed-bit Based Access Check
> -----------------------------------
>
> The implementation uses PTE Accessed-bit for basic access checks. That
> is, it clears the bit for the next sampling target page and checks
> whether it is set again after one sampling period. This could disturb
> the reclaim logic. DAMON uses ``PG_idle`` and ``PG_young`` page flags
> to solve the conflict, as Idle page tracking does.
>
> VMA-based Target Address Range Construction
> -------------------------------------------
>
> Only small parts in the super-huge virtual address space of the
> processes are mapped to physical memory and accessed. Thus, tracking
> the unmapped address regions is just wasteful. However, because DAMON
> can deal with some level of noise using the adaptive regions adjustment
> mechanism, tracking every mapping is not strictly required but could
> even incur a high overhead in some cases. That said, too huge unmapped
> areas inside the monitoring target should be removed to not take the
> time for the adaptive mechanism.
>
> For the reason, this implementation converts the complex mappings to
> three distinct regions that cover every mapped area of the address
> space. Also, the two gaps between the three regions are the two biggest
> unmapped areas in the given address space. The two biggest unmapped
> areas would be the gap between the heap and the uppermost mmap()-ed
> region, and the gap between the lowermost mmap()-ed region and the stack
> in most of the cases. Because these gaps are exceptionally huge in
> usual address spaces, excluding these will be sufficient to make a
> reasonable trade-off. Below shows this in detail::
>
> <heap>
> <BIG UNMAPPED REGION 1>
> <uppermost mmap()-ed region>
> (small mmap()-ed regions and munmap()-ed regions)
> <lowermost mmap()-ed region>
> <BIG UNMAPPED REGION 2>
> <stack>
>
> Signed-off-by: SeongJae Park <sjpark@xxxxxxxxx>
> Reviewed-by: Leonard Foerster <foersleo@xxxxxxxxx>
> Reviewed-by: Fernand Sieber <sieberf@xxxxxxxxxx>
Couple of nits below and one concern on the default value of
primitive_update_interval of virtual address space primitive.
Otherwise looks good to me.
[...]
> +
> +/*
> + * Size-evenly split a region into 'nr_pieces' small regions
> + *
> + * Returns 0 on success, or negative error code otherwise.
> + */
> +static int damon_va_evenly_split_region(struct damon_ctx *ctx,
I don't see ctx being used in this function.
> + struct damon_region *r, unsigned int nr_pieces)
> +{
> + unsigned long sz_orig, sz_piece, orig_end;
> + struct damon_region *n = NULL, *next;
> + unsigned long start;
> +
> + if (!r || !nr_pieces)
> + return -EINVAL;
> +
> + orig_end = r->ar.end;
> + sz_orig = r->ar.end - r->ar.start;
> + sz_piece = ALIGN_DOWN(sz_orig / nr_pieces, DAMON_MIN_REGION);
> +
> + if (!sz_piece)
> + return -EINVAL;
> +
> + r->ar.end = r->ar.start + sz_piece;
> + next = damon_next_region(r);
> + for (start = r->ar.end; start + sz_piece <= orig_end;
> + start += sz_piece) {
> + n = damon_new_region(start, start + sz_piece);
> + if (!n)
> + return -ENOMEM;
> + damon_insert_region(n, r, next);
> + r = n;
> + }
> + /* complement last region for possible rounding error */
> + if (n)
> + n->ar.end = orig_end;
> +
> + return 0;
> +}
[...]
> +/*
> + * Get the three regions in the given target (task)
> + *
> + * Returns 0 on success, negative error code otherwise.
> + */
> +static int damon_va_three_regions(struct damon_target *t,
> + struct damon_addr_range regions[3])
> +{
> + struct mm_struct *mm;
> + int rc;
> +
> + mm = damon_get_mm(t);
> + if (!mm)
> + return -EINVAL;
> +
> + mmap_read_lock(mm);
> + rc = __damon_va_three_regions(mm->mmap, regions);
> + mmap_read_unlock(mm);
This is being called for each target every second by default. Seems
too aggressive. Applications don't change their address space every
second. I would recommend to default ctx->primitive_update_interval to
a higher default value.
> +
> + mmput(mm);
> + return rc;
> +}
> +
[...]
> +static void __damon_va_init_regions(struct damon_ctx *c,
Keep the convention of naming damon_ctx ctx.
> + struct damon_target *t)
> +{
> + struct damon_region *r;
> + struct damon_addr_range regions[3];
> + unsigned long sz = 0, nr_pieces;
> + int i;
> +
> + if (damon_va_three_regions(t, regions)) {
> + pr_err("Failed to get three regions of target %lu\n", t->id);
> + return;
> + }
> +
> + for (i = 0; i < 3; i++)
> + sz += regions[i].end - regions[i].start;
> + if (c->min_nr_regions)
> + sz /= c->min_nr_regions;
> + if (sz < DAMON_MIN_REGION)
> + sz = DAMON_MIN_REGION;
> +
> + /* Set the initial three regions of the target */
> + for (i = 0; i < 3; i++) {
> + r = damon_new_region(regions[i].start, regions[i].end);
> + if (!r) {
> + pr_err("%d'th init region creation failed\n", i);
> + return;
> + }
> + damon_add_region(r, t);
> +
> + nr_pieces = (regions[i].end - regions[i].start) / sz;
> + damon_va_evenly_split_region(c, r, nr_pieces);
> + }
> +}
[...]
> +/*
> + * Update damon regions for the three big regions of the given target
> + *
> + * t the given target
> + * bregions the three big regions of the target
> + */
> +static void damon_va_apply_three_regions(struct damon_ctx *ctx,
ctx not used in this function.
> + struct damon_target *t, struct damon_addr_range bregions[3])
> +{
> + struct damon_region *r, *next;
> + unsigned int i = 0;
> +
> + /* Remove regions which are not in the three big regions now */
> + damon_for_each_region_safe(r, next, t) {
> + for (i = 0; i < 3; i++) {
> + if (damon_intersect(r, &bregions[i]))
> + break;
> + }
> + if (i == 3)
> + damon_destroy_region(r);
> + }
> +
> + /* Adjust intersecting regions to fit with the three big regions */
> + for (i = 0; i < 3; i++) {
> + struct damon_region *first = NULL, *last;
> + struct damon_region *newr;
> + struct damon_addr_range *br;
> +
> + br = &bregions[i];
> + /* Get the first and last regions which intersects with br */
> + damon_for_each_region(r, t) {
> + if (damon_intersect(r, br)) {
> + if (!first)
> + first = r;
> + last = r;
> + }
> + if (r->ar.start >= br->end)
> + break;
> + }
> + if (!first) {
> + /* no damon_region intersects with this big region */
> + newr = damon_new_region(
> + ALIGN_DOWN(br->start,
> + DAMON_MIN_REGION),
> + ALIGN(br->end, DAMON_MIN_REGION));
> + if (!newr)
> + continue;
> + damon_insert_region(newr, damon_prev_region(r), r);
> + } else {
> + first->ar.start = ALIGN_DOWN(br->start,
> + DAMON_MIN_REGION);
> + last->ar.end = ALIGN(br->end, DAMON_MIN_REGION);
> + }
> + }
> +}
