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Date:   Tue, 10 Mar 2020 08:57:21 +0000
From:   Jonathan Cameron <Jonathan.Cameron@Huawei.com>
To:     SeongJae Park <sjpark@amazon.com>
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Subject: Re: [PATCH v6 02/14] mm/damon: Implement region based sampling
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 <20200224123047.32506-3-sjpark@amazon.com>
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On Mon, 24 Feb 2020 13:30:35 +0100
SeongJae Park <sjpark@amazon.com> wrote:

> From: SeongJae Park <sjpark@amazon.de>
> 
> This commit implements DAMON's basic access check and region based
> sampling mechanisms.  This change would seems make no sense, mainly
> because it is only a part of the DAMON's logics.  Following two commits
> will make more sense.
> 
> This commit also exports `lookup_page_ext()` to GPL modules because
> DAMON uses the function but also supports the module build.

Do that as a separate patch before this one.  Makes it easy to spot.

> 
> Basic Access Check
> ------------------
> 
> DAMON basically reports what pages are how frequently accessed.  Note
> that the frequency is not an absolute number of accesses, but a relative
> frequency among the pages of the target workloads.
> 
> Users can control the resolution of the reports by setting two time
> intervals, ``sampling interval`` and ``aggregation interval``.  In
> detail, DAMON checks access to each page per ``sampling interval``,
> aggregates the results (counts the number of the accesses to each page),
> and reports the aggregated results per ``aggregation interval``.  For
> the access check of each page, DAMON uses the Accessed bits of PTEs.
> 
> This is thus similar to common periodic access checks based access
> tracking mechanisms, which overhead is increasing as the size of the
> target process grows.
> 
> Region Based Sampling
> ---------------------
> 
> To avoid the unbounded increase of the overhead, DAMON groups a number
> of adjacent pages that assumed to have same access frequencies into a
> region.  As long as the assumption (pages in a region have same access
> frequencies) is kept, only one page in the region is required to be
> checked.  Thus, for each ``sampling interval``, DAMON randomly picks one
> page in each region and clears its Accessed bit.  After one more
> ``sampling interval``, DAMON reads the Accessed bit of the page and
> increases the access frequency of the region if the bit has set
> meanwhile.  Therefore, the monitoring overhead is controllable by
> setting the number of regions.
> 
> Nonetheless, this scheme cannot preserve the quality of the output if
> the assumption is not kept.  Following commit will introduce how we can
> make the guarantee with best effort.
> 
> Signed-off-by: SeongJae Park <sjpark@amazon.de>

Various things inline. In particularly can you make use of standard
kthread_stop infrastructure rather than rolling your own?

> ---
>  mm/damon.c    | 509 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  mm/page_ext.c |   1 +
>  2 files changed, 510 insertions(+)
> 
> diff --git a/mm/damon.c b/mm/damon.c
> index aafdca35b7b8..6bdeb84d89af 100644
> --- a/mm/damon.c
> +++ b/mm/damon.c
> @@ -9,9 +9,14 @@
>  
>  #define pr_fmt(fmt) "damon: " fmt
>  
> +#include <linux/delay.h>
> +#include <linux/kthread.h>
>  #include <linux/mm.h>
>  #include <linux/module.h>
> +#include <linux/page_idle.h>
>  #include <linux/random.h>
> +#include <linux/sched/mm.h>
> +#include <linux/sched/task.h>
>  #include <linux/slab.h>
>  
>  #define damon_get_task_struct(t) \
> @@ -51,7 +56,24 @@ struct damon_task {
>  	struct list_head list;
>  };
>  
> +/*
> + * For each 'sample_interval', DAMON checks whether each region is accessed or
> + * not.  It aggregates and keeps the access information (number of accesses to
> + * each region) for each 'aggr_interval' time.
> + *
> + * All time intervals are in micro-seconds.
> + */
>  struct damon_ctx {
> +	unsigned long sample_interval;
> +	unsigned long aggr_interval;
> +	unsigned long min_nr_regions;
> +
> +	struct timespec64 last_aggregation;
> +
> +	struct task_struct *kdamond;
> +	bool kdamond_stop;
> +	spinlock_t kdamond_lock;
> +
>  	struct rnd_state rndseed;
>  
>  	struct list_head tasks_list;	/* 'damon_task' objects */
> @@ -204,6 +226,493 @@ static unsigned int nr_damon_regions(struct damon_task *t)
>  	return ret;
>  }
>  
> +/*
> + * Get the mm_struct of the given task
> + *
> + * Callser should put the mm_struct after use, unless it is NULL.

Caller 

> + *
> + * Returns the mm_struct of the task on success, NULL on failure
> + */
> +static struct mm_struct *damon_get_mm(struct damon_task *t)
> +{
> +	struct task_struct *task;
> +	struct mm_struct *mm;
> +
> +	task = damon_get_task_struct(t);
> +	if (!task)
> +		return NULL;
> +
> +	mm = get_task_mm(task);
> +	put_task_struct(task);
> +	return mm;
> +}
> +
> +/*
> + * Size-evenly split a region into 'nr_pieces' small regions
> + *
> + * Returns 0 on success, or negative error code otherwise.
> + */
> +static int damon_split_region_evenly(struct damon_ctx *ctx,
> +		struct damon_region *r, unsigned int nr_pieces)
> +{
> +	unsigned long sz_orig, sz_piece, orig_end;
> +	struct damon_region *piece = NULL, *next;
> +	unsigned long start;
> +
> +	if (!r || !nr_pieces)
> +		return -EINVAL;
> +
> +	orig_end = r->vm_end;
> +	sz_orig = r->vm_end - r->vm_start;
> +	sz_piece = sz_orig / nr_pieces;
> +
> +	if (!sz_piece)
> +		return -EINVAL;
> +
> +	r->vm_end = r->vm_start + sz_piece;
> +	next = damon_next_region(r);
> +	for (start = r->vm_end; start + sz_piece <= orig_end;
> +			start += sz_piece) {
> +		piece = damon_new_region(ctx, start, start + sz_piece);
> +		damon_add_region(piece, r, next);
> +		r = piece;
> +	}

I'd add a comment here. I think this next bit is to catch any rounding error
holes, but I'm not 100% sure.

> +	if (piece)
> +		piece->vm_end = orig_end;

blank line here.

> +	return 0;
> +}
> +
> +struct region {
> +	unsigned long start;
> +	unsigned long end;
> +};
> +
> +static unsigned long sz_region(struct region *r)
> +{
> +	return r->end - r->start;
> +}
> +
> +static void swap_regions(struct region *r1, struct region *r2)
> +{
> +	struct region tmp;
> +
> +	tmp = *r1;
> +	*r1 = *r2;
> +	*r2 = tmp;
> +}
> +
> +/*
> + * Find the three regions in an address space
> + *
> + * vma		the head vma of the target address space
> + * regions	an array of three 'struct region's that results will be saved
> + *
> + * This function receives an address space and finds three regions in it which
> + * separated by the two biggest unmapped regions in the space.  Please refer to
> + * below comments of 'damon_init_regions_of()' function to know why this is
> + * necessary.
> + *
> + * Returns 0 if success, or negative error code otherwise.
> + */
> +static int damon_three_regions_in_vmas(struct vm_area_struct *vma,
> +		struct region regions[3])
> +{
> +	struct region gap = {0,}, first_gap = {0,}, second_gap = {0,};
> +	struct vm_area_struct *last_vma = NULL;
> +	unsigned long start = 0;
> +
> +	/* Find two biggest gaps so that first_gap > second_gap > others */
> +	for (; vma; vma = vma->vm_next) {
> +		if (!last_vma) {
> +			start = vma->vm_start;
> +			last_vma = vma;
> +			continue;
> +		}
> +		gap.start = last_vma->vm_end;
> +		gap.end = vma->vm_start;
> +		if (sz_region(&gap) > sz_region(&second_gap)) {
> +			swap_regions(&gap, &second_gap);
> +			if (sz_region(&second_gap) > sz_region(&first_gap))
> +				swap_regions(&second_gap, &first_gap);
> +		}
> +		last_vma = vma;
> +	}
> +
> +	if (!sz_region(&second_gap) || !sz_region(&first_gap))
> +		return -EINVAL;
> +
> +	/* Sort the two biggest gaps by address */
> +	if (first_gap.start > second_gap.start)
> +		swap_regions(&first_gap, &second_gap);
> +
> +	/* Store the result */
> +	regions[0].start = start;
> +	regions[0].end = first_gap.start;
> +	regions[1].start = first_gap.end;
> +	regions[1].end = second_gap.start;
> +	regions[2].start = second_gap.end;
> +	regions[2].end = last_vma->vm_end;
> +
> +	return 0;
> +}
> +
> +/*
> + * Get the three regions in the given task
> + *
> + * Returns 0 on success, negative error code otherwise.
> + */
> +static int damon_three_regions_of(struct damon_task *t,
> +				struct region regions[3])
> +{
> +	struct mm_struct *mm;
> +	int ret;
> +
> +	mm = damon_get_mm(t);
> +	if (!mm)
> +		return -EINVAL;
> +
> +	down_read(&mm->mmap_sem);
> +	ret = damon_three_regions_in_vmas(mm->mmap, regions);
> +	up_read(&mm->mmap_sem);
> +
> +	mmput(mm);
> +	return ret;
> +}
> +
> +/*
> + * Initialize the monitoring target regions for the given task
> + *
> + * t	the given target task
> + *
> + * Because only a number of small portions of the entire address space
> + * is acutally mapped to the memory and accessed, monitoring the unmapped

actually

> + * regions is wasteful.  That said, because we can deal with small noises,
> + * tracking every mapping is not strictly required but could even incur a high
> + * overhead if the mapping frequently changes or the number of mappings is
> + * high.  Nonetheless, this may seems very weird.  DAMON's dynamic regions
> + * adjustment mechanism, which will be implemented with following commit will
> + * make this more sense.
> + *
> + * For the reason, we convert the complex mappings to three distinct regions
> + * that cover every mapped areas of the address space.  Also the two gaps
> + * between the three regions are the two biggest unmapped areas in the given
> + * address space.  In detail, this function first identifies the start and the
> + * end of the mappings and the two biggest unmapped areas of the address space.
> + * Then, it constructs the three regions as below:
> + *
> + *     [mappings[0]->start, big_two_unmapped_areas[0]->start)
> + *     [big_two_unmapped_areas[0]->end, big_two_unmapped_areas[1]->start)
> + *     [big_two_unmapped_areas[1]->end, mappings[nr_mappings - 1]->end)
> + *
> + * As usual memory map of processes is as below, the gap between the heap and
> + * the uppermost mmap()-ed region, and the gap between the lowermost mmap()-ed
> + * region and the stack will be two biggest unmapped regions.  Because these
> + * gaps are exceptionally huge areas in usual address space, excluding these
> + * two biggest unmapped regions will be sufficient to make a trade-off.
> + *
> + *   <heap>
> + *   <BIG UNMAPPED REGION 1>
> + *   <uppermost mmap()-ed region>
> + *   (other mmap()-ed regions and small unmapped regions)
> + *   <lowermost mmap()-ed region>
> + *   <BIG UNMAPPED REGION 2>
> + *   <stack>
> + */
> +static void damon_init_regions_of(struct damon_ctx *c, struct damon_task *t)
> +{
> +	struct damon_region *r;
> +	struct region regions[3];
> +	int i;
> +
> +	if (damon_three_regions_of(t, regions)) {
> +		pr_err("Failed to get three regions of task %lu\n", t->pid);
> +		return;
> +	}
> +
> +	/* Set the initial three regions of the task */
> +	for (i = 0; i < 3; i++) {
> +		r = damon_new_region(c, regions[i].start, regions[i].end);
> +		damon_add_region_tail(r, t);
> +	}
> +
> +	/* Split the middle region into 'min_nr_regions - 2' regions */
> +	r = damon_nth_region_of(t, 1);
> +	if (damon_split_region_evenly(c, r, c->min_nr_regions - 2))
> +		pr_warn("Init middle region failed to be split\n");
> +}
> +
> +/* Initialize '->regions_list' of every task */
> +static void kdamond_init_regions(struct damon_ctx *ctx)
> +{
> +	struct damon_task *t;
> +
> +	damon_for_each_task(ctx, t)
> +		damon_init_regions_of(ctx, t);
> +}
> +
> +/*
> + * Check whether the given region has accessed since the last check

Should also make clear that this sets us up for the next access check at
a different memory address it the region.

Given the lack of connection between activities perhaps just split this into
two functions that are always called next to each other.

> + *
> + * mm	'mm_struct' for the given virtual address space
> + * r	the region to be checked
> + */
> +static void kdamond_check_access(struct damon_ctx *ctx,
> +			struct mm_struct *mm, struct damon_region *r)
> +{
> +	pte_t *pte = NULL;
> +	pmd_t *pmd = NULL;
> +	spinlock_t *ptl;
> +
> +	if (follow_pte_pmd(mm, r->sampling_addr, NULL, &pte, &pmd, &ptl))
> +		goto mkold;
> +
> +	/* Read the page table access bit of the page */
> +	if (pte && pte_young(*pte))
> +		r->nr_accesses++;
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE

Is it worth having this protection?  Seems likely to have only a very small
influence on performance and makes it a little harder to reason about the code.

> +	else if (pmd && pmd_young(*pmd))
> +		r->nr_accesses++;
> +#endif	/* CONFIG_TRANSPARENT_HUGEPAGE */
> +
> +	spin_unlock(ptl);
> +
> +mkold:
> +	/* mkold next target */
> +	r->sampling_addr = damon_rand(ctx, r->vm_start, r->vm_end);
> +
> +	if (follow_pte_pmd(mm, r->sampling_addr, NULL, &pte, &pmd, &ptl))
> +		return;
> +
> +	if (pte) {
> +		if (pte_young(*pte)) {
> +			clear_page_idle(pte_page(*pte));
> +			set_page_young(pte_page(*pte));
> +		}
> +		*pte = pte_mkold(*pte);
> +	}
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +	else if (pmd) {
> +		if (pmd_young(*pmd)) {
> +			clear_page_idle(pmd_page(*pmd));
> +			set_page_young(pmd_page(*pmd));
> +		}
> +		*pmd = pmd_mkold(*pmd);
> +	}
> +#endif
> +
> +	spin_unlock(ptl);
> +}
> +
> +/*
> + * Check whether a time interval is elapsed

Another comment block that would be clearer if it was kernel-doc rather
than nearly kernel-doc

> + *
> + * baseline	the time to check whether the interval has elapsed since
> + * interval	the time interval (microseconds)
> + *
> + * See whether the given time interval has passed since the given baseline
> + * time.  If so, it also updates the baseline to current time for next check.
> + *
> + * Returns true if the time interval has passed, or false otherwise.
> + */
> +static bool damon_check_reset_time_interval(struct timespec64 *baseline,
> +		unsigned long interval)
> +{
> +	struct timespec64 now;
> +
> +	ktime_get_coarse_ts64(&now);
> +	if ((timespec64_to_ns(&now) - timespec64_to_ns(baseline)) <
> +			interval * 1000)
> +		return false;
> +	*baseline = now;
> +	return true;
> +}
> +
> +/*
> + * Check whether it is time to flush the aggregated information
> + */
> +static bool kdamond_aggregate_interval_passed(struct damon_ctx *ctx)
> +{
> +	return damon_check_reset_time_interval(&ctx->last_aggregation,
> +			ctx->aggr_interval);
> +}
> +
> +/*
> + * Reset the aggregated monitoring results
> + */
> +static void kdamond_flush_aggregated(struct damon_ctx *c)

I wouldn't expect a reset function to be called flush.

> +{
> +	struct damon_task *t;
> +	struct damon_region *r;
> +
> +	damon_for_each_task(c, t) {
> +		damon_for_each_region(r, t)
> +			r->nr_accesses = 0;
> +	}
> +}
> +
> +/*
> + * Check whether current monitoring should be stopped
> + *
> + * If users asked to stop, need stop.  Even though no user has asked to stop,
> + * need stop if every target task has dead.
> + *
> + * Returns true if need to stop current monitoring.
> + */
> +static bool kdamond_need_stop(struct damon_ctx *ctx)
> +{
> +	struct damon_task *t;
> +	struct task_struct *task;
> +	bool stop;
> +

As below comment asks, can you use kthread_should_stop?

> +	spin_lock(&ctx->kdamond_lock);
> +	stop = ctx->kdamond_stop;
> +	spin_unlock(&ctx->kdamond_lock);
> +	if (stop)
> +		return true;
> +
> +	damon_for_each_task(ctx, t) {
> +		task = damon_get_task_struct(t);
> +		if (task) {
> +			put_task_struct(task);
> +			return false;
> +		}
> +	}
> +
> +	return true;
> +}
> +
> +/*
> + * The monitoring daemon that runs as a kernel thread
> + */
> +static int kdamond_fn(void *data)
> +{
> +	struct damon_ctx *ctx = (struct damon_ctx *)data;

Never any need to explicitly cast a void * to some other pointer type.
(C spec)

	struct damon_ctx *ctx = data;
> +	struct damon_task *t;
> +	struct damon_region *r, *next;
> +	struct mm_struct *mm;
> +
> +	pr_info("kdamond (%d) starts\n", ctx->kdamond->pid);
> +	kdamond_init_regions(ctx);
> +	while (!kdamond_need_stop(ctx)) {
> +		damon_for_each_task(ctx, t) {
> +			mm = damon_get_mm(t);
> +			if (!mm)
> +				continue;
> +			damon_for_each_region(r, t)
> +				kdamond_check_access(ctx, mm, r);
> +			mmput(mm);
> +		}
> +
> +		if (kdamond_aggregate_interval_passed(ctx))
> +			kdamond_flush_aggregated(ctx);
> +
> +		usleep_range(ctx->sample_interval, ctx->sample_interval + 1);

Is there any purpose in using a range for such a narrow window?

> +	}
> +	damon_for_each_task(ctx, t) {
> +		damon_for_each_region_safe(r, next, t)
> +			damon_destroy_region(r);
> +	}
> +	pr_info("kdamond (%d) finishes\n", ctx->kdamond->pid);

Feels like noise.  I'd drop tis to pr_debug.

> +	spin_lock(&ctx->kdamond_lock);
> +	ctx->kdamond = NULL;
> +	spin_unlock(&ctx->kdamond_lock);

blank line.

> +	return 0;
> +}
> +
> +/*
> + * Controller functions
> + */
> +
> +/*
> + * Start or stop the kdamond
> + *
> + * Returns 0 if success, negative error code otherwise.
> + */
> +static int damon_turn_kdamond(struct damon_ctx *ctx, bool on)
> +{
> +	spin_lock(&ctx->kdamond_lock);
> +	ctx->kdamond_stop = !on;

Can't use the kthread_stop / kthread_should_stop approach?

> +	if (!ctx->kdamond && on) {
> +		ctx->kdamond = kthread_run(kdamond_fn, ctx, "kdamond");
> +		if (!ctx->kdamond)
> +			goto fail;
> +		goto success;

cleaner as 
int ret = 0; above then

		if (!ctx->kdamond)
			ret = -EINVAL;
		goto unlock;

with

unlock:
	spin_unlock(&ctx->dmanond_lock);
	return ret;

> +	}
> +	if (ctx->kdamond && !on) {
> +		spin_unlock(&ctx->kdamond_lock);
> +		while (true) {

An unbounded loop is probably a bad idea.

> +			spin_lock(&ctx->kdamond_lock);
> +			if (!ctx->kdamond)
> +				goto success;
> +			spin_unlock(&ctx->kdamond_lock);
> +
> +			usleep_range(ctx->sample_interval,
> +					ctx->sample_interval * 2);
> +		}
> +	}
> +
> +	/* tried to turn on while turned on, or turn off while turned off */
> +
> +fail:
> +	spin_unlock(&ctx->kdamond_lock);
> +	return -EINVAL;
> +
> +success:
> +	spin_unlock(&ctx->kdamond_lock);
> +	return 0;
> +}
> +
> +/*
> + * This function should not be called while the kdamond is running.
> + */
> +static int damon_set_pids(struct damon_ctx *ctx,
> +			unsigned long *pids, ssize_t nr_pids)
> +{
> +	ssize_t i;
> +	struct damon_task *t, *next;
> +
> +	damon_for_each_task_safe(ctx, t, next)
> +		damon_destroy_task(t);
> +
> +	for (i = 0; i < nr_pids; i++) {
> +		t = damon_new_task(pids[i]);
> +		if (!t) {
> +			pr_err("Failed to alloc damon_task\n");
> +			return -ENOMEM;
> +		}
> +		damon_add_task_tail(ctx, t);
> +	}
> +
> +	return 0;
> +}
> +
> +/*

This is kind of similar to kernel-doc formatting.  Might as well just make
it kernel-doc!

> + * Set attributes for the monitoring
> + *
> + * sample_int		time interval between samplings
> + * aggr_int		time interval between aggregations
> + * min_nr_reg		minimal number of regions
> + *
> + * This function should not be called while the kdamond is running.
> + * Every time interval is in micro-seconds.
> + *
> + * Returns 0 on success, negative error code otherwise.
> + */
> +static int damon_set_attrs(struct damon_ctx *ctx, unsigned long sample_int,
> +		unsigned long aggr_int, unsigned long min_nr_reg)
> +{
> +	if (min_nr_reg < 3) {
> +		pr_err("min_nr_regions (%lu) should be bigger than 2\n",
> +				min_nr_reg);
> +		return -EINVAL;
> +	}
> +
> +	ctx->sample_interval = sample_int;
> +	ctx->aggr_interval = aggr_int;
> +	ctx->min_nr_regions = min_nr_reg;

blank line helps readability a tiny little bit.

> +	return 0;
> +}
> +
>  static int __init damon_init(void)
>  {
>  	pr_info("init\n");
> diff --git a/mm/page_ext.c b/mm/page_ext.c
> index 4ade843ff588..71169b45bba9 100644
> --- a/mm/page_ext.c
> +++ b/mm/page_ext.c
> @@ -131,6 +131,7 @@ struct page_ext *lookup_page_ext(const struct page *page)
>  					MAX_ORDER_NR_PAGES);
>  	return get_entry(base, index);
>  }
> +EXPORT_SYMBOL_GPL(lookup_page_ext);
>  
>  static int __init alloc_node_page_ext(int nid)
>  {