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Message-ID: <9f81f89f-c3ad-4cef-a619-ad36348c8ef5@redhat.com>
Date:   Tue, 12 Dec 2023 14:44:04 +0100
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH v3 1/4] mm/ksm: add ksm advisor
Content-Language: en-US
To:     Stefan Roesch <shr@devkernel.io>, kernel-team@fb.com
Cc:     akpm@linux-foundation.org, hannes@cmpxchg.org, riel@surriel.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
References: <20231204234906.1237478-1-shr@devkernel.io>
 <20231204234906.1237478-2-shr@devkernel.io>
From:   David Hildenbrand <david@redhat.com>
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In-Reply-To: <20231204234906.1237478-2-shr@devkernel.io>
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[...]

> +
> +/**
> + * struct advisor_ctx - metadata for KSM advisor
> + * @start_scan: start time of the current scan
> + * @scan_time: scan time of previous scan
> + * @change: change in percent to pages_to_scan parameter
> + * @cpu_time: cpu time consumed by the ksmd thread in the previous scan
> + */
> +struct advisor_ctx {
> +	ktime_t start_scan;
> +	unsigned long scan_time;
> +	unsigned long change;
> +	unsigned long long cpu_time;
> +};
> +static struct advisor_ctx advisor_ctx;
> +
> +/* Define different advisor's */
> +enum ksm_advisor_type {
> +	KSM_ADVISOR_NONE,
> +	KSM_ADVISOR_SCAN_TIME,
> +};
> +static enum ksm_advisor_type ksm_advisor;
> +
> +static void init_advisor(void)
> +{
> +	advisor_ctx = (const struct advisor_ctx){ 0 };
> +}

Again, you can drop this completely. The static values are already 
initialized to 0.

Or is there any reason to initialize to 0 explicitly?

> +
> +static void set_advisor_defaults(void)
> +{
> +	if (ksm_advisor == KSM_ADVISOR_NONE)
> +		ksm_thread_pages_to_scan = DEFAULT_PAGES_TO_SCAN;
> +	else if (ksm_advisor == KSM_ADVISOR_SCAN_TIME)
> +		ksm_thread_pages_to_scan = ksm_advisor_min_pages;
> +}

That function is unused?

> +
> +static inline void advisor_start_scan(void)
> +{
> +	if (ksm_advisor == KSM_ADVISOR_SCAN_TIME)
> +		advisor_ctx.start_scan = ktime_get();
> +}
> +
> +static inline s64 advisor_stop_scan(void)
> +{
> +	return ktime_ms_delta(ktime_get(), advisor_ctx.start_scan);
> +}

Just inline that into the caller. Then rename run_advisor() into 
advisor_stop_scan(). So in scan_get_next_rmap_item)( you have paired 
start+stop hooks.

> +
> +/*
> + * Use previous scan time if available, otherwise use current scan time as an
> + * approximation for the previous scan time.
> + */
> +static inline unsigned long prev_scan_time(struct advisor_ctx *ctx,
> +					   unsigned long scan_time)
> +{
> +	return ctx->scan_time ? ctx->scan_time : scan_time;
> +}
> +
> +/* Calculate exponential weighted moving average */
> +static unsigned long ewma(unsigned long prev, unsigned long curr)
> +{
> +	return ((100 - EWMA_WEIGHT) * prev + EWMA_WEIGHT * curr) / 100;
> +}
> +
> +/*
> + * The scan time advisor is based on the current scan rate and the target
> + * scan rate.
> + *
> + *      new_pages_to_scan = pages_to_scan * (scan_time / target_scan_time)
> + *
> + * To avoid pertubations it calculates a change factor of previous changes.

s/pertubations/perturbations/

Do you also want to describe how min/max CPU comes into play?

> + * A new change factor is calculated for each iteration and it uses an
> + * exponentially weighted moving average. The new pages_to_scan value is
> + * multiplied with that change factor:
> + *
> + *      new_pages_to_scan *= change facor
> + *
> + * In addition the new pages_to_scan value is capped by the max and min
> + * limits.
> + */


With that, LGTM

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb