tsacct: skip all kernel threads

This patch is a preparation step for HPCC, for the OOM killer
improvements.  I suspect that this patch is useful on its own, because it
really makes no sense to sum up accounting statistics of use_mm within
kernel threads which are only temporarily using those mm.

When we hit acct_account_cputime within a irq handler over a kthread that
happens to use a userspace mm, we end up summing up the mm's RSS into the
tsk acct_rss_mem1, which eventually decays.

I don't see a good rationale behind tracking the mm's rss in that way when
a kthread use a userspace mm temporarily through use_mm.

It causes issues with init_mm and efi_mm which only partially initialize
their mm_struct when introducing the new hierarchical percpu counters to
replace RSS counters, which requires a pointer dereference when reading
the approximate counter sum.  The current percpu counters simply load a
zeroed atomic counter, which happen to work.

Skip all kernel threads in acct_account_cputime(), not just those that
happen to have a NULL mm.

This is a preparation step before introducing the hierarchical percpu
counters.

Link: https://lkml.kernel.org/r/20251224173810.648699-2-mathieu.desnoyers@efficios.com
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Aboorva Devarajan <aboorvad@linux.ibm.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Christan König <christian.koenig@amd.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: "Liam R . Howlett" <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Martin Liu <liumartin@google.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: SeongJae Park <sj@kernel.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Cc: Tejun Heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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