Re: [RFC PATCH] mm: net: disable kswapd for high-order network buffer allocation

On Tue, Oct 14, 2025 at 1:17 AM Barry Song [off-list ref] wrote:

On Tue, Oct 14, 2025 at 3:01 PM Eric Dumazet [off-list ref] wrote:

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On Mon, Oct 13, 2025 at 11:43 PM Barry Song [off-list ref] wrote:

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A problem with the existing sysctl is that it only covers the TX path;
for the RX path, we also observe that kswapd consumes significant power.
I could add the patch below to make it support the RX path, but it feels
like a bit of a layer violation, since the RX path code resides in mm
and is intended to serve generic users rather than networking, even
though the current callers are primarily network-related.

You might have a buggy driver.

We are observing the RX path as follows:

do_softirq
    taskset_hi_action
       kalPacketAlloc
           __netdev_alloc_skb
               page_frag_alloc_align
                   __page_frag_cache_refill

This appears to be a fairly common stack.

So it is a buggy driver?

No idea, kalPacketAlloc is not in upstream trees.

It apparently needs high order allocations. It will fail at some point.

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High performance drivers use order-0 allocations only.

Do you have an example of high-performance drivers that use only order-0 memory?

About all drivers using XDP, and/or using napi_get_frags()

XDP has been using order-0 pages from the very beginning.

Thanks! But there are still many drivers using netdev_alloc_skb()—we
shouldn’t overlook them, right?

net % git grep netdev_alloc_skb | wc -l
     359

Only the ones that are using 16KB allocations like some WAN drivers :)

Some networks use MTU=9000

If a hardware does not provide SG support on receive, a kmalloc()
based will use 16KB of memory.

By using a frag allocator, we can pack 3 allocations per 32KB instead of 2.

TCP can go 50% faster.

If memory is short, it will fail no matter what.