On Thu, 2 Apr 2020 18:24:01 +0100 Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> wrote: > On Thu, 2 Apr 2020 15:59:59 +0200 > SeongJae Park <sjpark@xxxxxxxxxx> wrote: > > > On Wed, 1 Apr 2020 10:22:22 +0200 SeongJae Park <sjpark@xxxxxxxxxx> wrote: > > > > > On Tue, 31 Mar 2020 17:02:33 +0100 Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> wrote: > > > > > > > On Wed, 18 Mar 2020 12:27:11 +0100 > > > > SeongJae Park <sjpark@xxxxxxxxxx> wrote: > > > > > > > > > From: SeongJae Park <sjpark@xxxxxxxxx> > > > > > > > > > > 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. > > > > > > > > > > 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@xxxxxxxxx> > > > > > > > > Hi. > > > > > > > > A few comments inline. > > > > > > > > I've still not replicated your benchmarks so may well have some more > > > > feedback once I've managed that on one of our servers. > > > > > > Appreciate your comments. If you need any help for the replication, please let > > > me know. I basically use my parsec3 wrapper scripts[1] to run parsec3 and > > > splash2x workloads and `damo` tool, which resides in the kernel tree at > > > `/tools/damon/`. > > > > > > For example, below commands will reproduce ethp applied splash2x/fft run. > > > > > > $ echo "2M null 5 null null null hugepage > > > 2M null null 5 1s null nohugepage" > ethp > > > $ parsec3_on_ubuntu/run.sh splash2x.fft > > > $ linux/tools/damon/damo schemes -c ethp `pidof fft` > > > > > > [1] https://github.com/sjp38/parsec3_on_ubuntu > > > > > > > > > > > Thanks, > > > > > > > > Jonathan > > > > > > > > > --- > > > > > include/linux/damon.h | 24 ++ > > > > > mm/damon.c | 553 ++++++++++++++++++++++++++++++++++++++++++ > > > > > 2 files changed, 577 insertions(+) > > > > > > > > [...] > > > > > diff --git a/mm/damon.c b/mm/damon.c > > > > > index d7e6226ab7f1..018016793555 100644 > > > > > --- a/mm/damon.c > > > > > +++ b/mm/damon.c > > > > > @@ -10,8 +10,14 @@ > > > > > #define pr_fmt(fmt) "damon: " fmt > > > > > > > > > > #include <linux/damon.h> > > > > > +#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> > > > > > > > > [...] > > > > > +/* > > > > > + * 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); > > > > piece may be n > > > > > > Yes, that name is short and more intuitive. I will rename so. > > > > > > > > + damon_insert_region(piece, r, next); > > > > > + r = piece; > > > > > + } > > > > > + /* complement last region for possible rounding error */ > > > > > + if (piece) > > > > > + piece->vm_end = orig_end; > > > > > > > > Update the sampling address to ensure it's in the region? > > > > > > I think `piece->vm_end` should be equal or smaller than `orig_end` and > > > therefore the sampling address of `piece` will be still in the region. > > > > > > > > > > > > + > > > > > + return 0; > > > > > +} > > > > > + > > > [...] > > > > > +static void damon_pte_pmd_mkold(pte_t *pte, pmd_t *pmd) > > > > > +{ > > > > > + if (pte) { > > > > > + if (pte_young(*pte)) { > > > > > + clear_page_idle(pte_page(*pte)); > > > > > + set_page_young(pte_page(*pte)); > > > > > + } > > > > > + *pte = pte_mkold(*pte); > > > > > + return; > > > > > + } > > > > > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE > > > > > + if (pmd) { > > > > > + if (pmd_young(*pmd)) { > > > > > + clear_page_idle(pmd_page(*pmd)); > > > > > + set_page_young(pmd_page(*pmd)); > > > > > + } > > > > > + *pmd = pmd_mkold(*pmd); > > > > > + } > > > > > +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */ > > > > > > > > No need to flush the TLBs? > > > > > > Good point! > > > > > > I have intentionally skipped TLB flushing here to minimize the performance > > > effect to the target workload. I also thought this might not degrade the > > > monitoring accuracy so much because we are targetting for the DRAM level > > > accesses of memory-intensive workloads, which might make TLB flood frequently. > > > > > > However, your comment makes me thinking differently now. By flushing the TLB > > > here, we will increase up to `number_of_regions` TLB misses for sampling > > > interval. This might be not a huge overhead. Also, improving the monitoring > > > accuracy makes no harm at all. I even didn't measured the overhead. > > > > > > I will test the overhead and if it is not significant, I will make this code to > > > flush TLB, in the next spin. > > > > Hmm, it seems like 'page_idle.c' is also modifying the Accessed bit but doesn't > > flush related TLB entries. If I'm not missing something here, I would like to > > leave this part as is to make the behavior consistent. > > Interesting. In that usecase, the risk is that the MMU believes > the page still has the accessed bit set when we have cleared it and hence > the accessed bit is not written out to the table in memory. > > That will give them a wrong decision so not great and would lead to them > thinking more pages are idle than are. > > Here we could have a particular TLB entry for a huge page in which > a region lies entirely. Because we don't flush the TLB each time > we could end with a count of 0 accesses when it should be the maximum. > A very frequently accessed page might well sit in the TLB for a very > long time (particularly if the TLB is running a clever eviction > strategy). > > I think we would want to be test this and see if we get that > pathological case sometimes. Also worth benchmarking if it actually > costs us very much to do the flushes. Agreed, it wouldn't be late to make a decision after measuring the real cost. I will share the measurement results soon. Meanwhile, it would be helpful if anyone could confirm whether page_idle.c is skipping TLB flushing and explain why such decision has made. Thanks, SeongJae Park > > Jonathan > > > > > > > Thanks, > > SeongJae Park > > > > > > > > > > > > > > +} > > > > > + > > > [...] > > > > > +/* > > > > > + * The monitoring daemon that runs as a kernel thread > > > > > + */ > > > > > +static int kdamond_fn(void *data) > > > > > +{ > > > > > + 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); > > > > > > > > We haven't called mkold on the initial regions so first check will > > > > get us fairly random state. > > > > > > Yes, indeed. However, the early results will not be accurate anyway because > > > the adaptive regions adjustment algorithm will not take effect yet. I would > > > like to leave this part as is but add some comments about this point to keep > > > the code simple. > > > > > > > > > > > > + 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_reset_aggregated(ctx); > > > > > + > > > > > + usleep_range(ctx->sample_interval, ctx->sample_interval + 1); > > > > > + } > > > > > + damon_for_each_task(ctx, t) { > > > > > + damon_for_each_region_safe(r, next, t) > > > > > + damon_destroy_region(r); > > > > > + } > > > > > + pr_debug("kdamond (%d) finishes\n", ctx->kdamond->pid); > > > > > + mutex_lock(&ctx->kdamond_lock); > > > > > + ctx->kdamond = NULL; > > > > > + mutex_unlock(&ctx->kdamond_lock); > > > > > + > > > > > + return 0; > > > > > +} > > > > > + > > > [...] > > > > > +/* > > > > > + * Start or stop the kdamond > > > > > + * > > > > > + * Returns 0 if success, negative error code otherwise. > > > > > + */ > > > > > +static int damon_turn_kdamond(struct damon_ctx *ctx, bool on) > > > > > +{ > > > > > + int err = -EBUSY; > > > > > + > > > > > + mutex_lock(&ctx->kdamond_lock); > > > > > + if (!ctx->kdamond && on) { > > > > > > > > Given there is very little shared code between on and off, I would > > > > suggest just splitting it into two functions. > > > > > > Good point, I will do so in next spin. > > > > > > > > > > > > + err = 0; > > > > > + ctx->kdamond = kthread_run(kdamond_fn, ctx, "kdamond"); > > > > > + if (IS_ERR(ctx->kdamond)) > > > > > + err = PTR_ERR(ctx->kdamond); > > > > > + } else if (ctx->kdamond && !on) { > > > > > + mutex_unlock(&ctx->kdamond_lock); > > > > > + kthread_stop(ctx->kdamond); > > > > > + while (damon_kdamond_running(ctx)) > > > > > + usleep_range(ctx->sample_interval, > > > > > + ctx->sample_interval * 2); > > > > > + return 0; > > > > > + } > > > > > + mutex_unlock(&ctx->kdamond_lock); > > > > > + > > > > > + return err; > > > > > +} > > > > > + > > > [...] > > > > > + > > > > > +/* > > > > > > > > Why not make these actual kernel-doc? That way you can use the > > > > kernel-doc scripts to sanity check them. > > > > > > Oops, I just forgot that it should start with '/**'. Will fix it in next spin. > > > > > > > > > Thanks, > > > SeongJae Park > > > > > > > > > > > /** > > > > > > > > > + * damon_set_attrs() - Set attributes for the monitoring. > > > > > + * @ctx: monitoring context > > > > > + * @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. > > > > > + * > > > > > + * Return: 0 on success, negative error code otherwise. > > > > > + */ > > > > > +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; > > > > > + > > > > > + return 0; > > > > > +} > > > > > + > > > > > static int __init damon_init(void) > > > > > { > > > > > return 0; > > > > > >
