linux-kernel - Re: [RFC PATCH v2 4/4] Allow to trace fd usage with rlimit-events

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Date:   Thu, 14 Dec 2017 16:59:23 -0700
From:   Andreas Dilger <adilger@...ger.ca>
To:     Krzysztof Opasiak <k.opasiak@...sung.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-arch@...r.kernel.org,
        k.lewandowsk@...sung.com, l.stelmach@...sung.com,
        p.szewczyk@...sung.com
Subject: Re: [RFC PATCH v2 4/4] Allow to trace fd usage with rlimit-events

On Dec 14, 2017, at 3:00 PM, Krzysztof Opasiak <k.opasiak@...sung.com> wrote:
> 
> Add rlimit-events calls to file descriptors management
> code to allow tracing of FD usage.
> 
> This allows userspace process (monitor) to get notification when
> other process (subject) uses given amount of file descriptors.
> 
> This can be used to for example asynchronously monitor number
> of open FD's in system services instead of polling with
> predefined interval.

I'm not involved in this area of code, but one optimization question:

> +static unsigned int count_open_fds(struct fdtable *fdt)
> +{
> +	unsigned int maxfd = fdt->max_fds;
> +	unsigned int maxbit = maxfd / BITS_PER_LONG;
> +	unsigned int count = 0;
> +	int i;
> +
> +	i = find_next_zero_bit(fdt->full_fds_bits, maxbit, 0);
> +	/* If there is no free fds */
> +	if (i > maxbit)
> +		return maxfd;
> +#if BITS_PER_LONG == 32
> +#define HWEIGHT_LONG hweight32
> +#else
> +#define HWEIGHT_LONG hweight64
> +#endif
> +
> +	count += i * BITS_PER_LONG;
> +	for (; i < maxbit; ++i)
> +		count += HWEIGHT_LONG(fdt->open_fds[i]);
> +
> +#undef HWEIGHT_LONG
> +	return count;
> +}

Since find_next_zero_bit() needs to process all of the words anyway
as well as lots of extra operations that add overhead, it looks more
efficient to just compute HWEIGHT_LONG(open_fds[]) for the whole array.

Cheers, Andreas






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